I'm a contractor/carpenter with some hardwood flooring experience. I bought a 1880's house from another contractor who didn't finish the rebuild. There's a living room next to a dining room that concern me. Before building the stud wall between these 2 rooms, and their exterior stud walls, the past owner put down 1/2" ply over the original 7/8" th. sap-pine T&G subfloor, so the floor is pretty firm.
But it's not flat (nor level, but that's to be expected). Over the approx. 16' x 25' that's in these 2 rooms the sub floor moves up and down maybe 1.5", and some of the dips are pretty noticeable.
I'm thinking of putting down a 3/4" herringbone pattern floor so I'd like to deal with the flatness issue of the sub floor first. I've tried, unsuccessfully, to find a self leveling pourable product that when dry could be nailed through. Nor have I found a trowelable product I could nail through after it's set up.
I'd rather not add any additional layers to the existing sub floor because of the offset that would be created between these rooms and the others, and taking off the 1/2" ply would be a job since it runs under walls and I think it is glued and nailed down.
How flat do I need for a herringbone pattern floor (pieces would be around 2.5" x 18") and how can I build up the low areas with something other then felt paper, which I don't think would work for more then 3/16"?

Your mistake is thinking you can nail down the herringbone in the first place

With herringbone or chevron, you should be doing a full spread glue down install. Nailing just will not keep the planks together and you'll have gaps everywhere. Herringbone is one of those things that NEEDS to be as tight as possible going in and if the humidity drops below 30% at all your going to get an ugly gapping mess and will be disappointed in your expensive purchase.

Take the time and spend a bit more to do it right - herringbone also requires flatness to 1/8th over 10ft; if you install over what you describe, you'll have gaps and deflection everywhere. IMO, build up the lowest points with tar paper or shims and then skim coat the area with self leveller to get it flat. I did a chevron install in a historic building that was converted to a commercial space a couple years back that was over plywood that was as bad as you describe. We didn't have an option to fix the subfloor so I ended up putting mesh in to reinforce the concrete leveller that we used and they solution worked perfectly. I had some concerns about the extra weight but it ended up being a non-issue.

What is 'shelf leveller'?
So, you're saying you'd lay a whole floor, 3/4 solid wood, with out nailing it at all? In my limited experience I've had 20" long pieces that were less then straight that I could straighten, so to speak, by nailing them
This is a honest question----if the glue is so good, why uses nails at all, ever? I'm surprised to think that glue can keep wood from moving. The expansion/contraction that takes place is a huge force. I've seen many parquet floors that have failed (but I acknowledge that could be caused by many different factors, not just glue)

Shelf leveller is probably a typo

Yes you can use nails to straighten out crooked boards when you install - I prefer not to use bad material but sometimes you get stuck having to improvise. For 90% of our installs we use nails, because the cost is much less than a full trowel glue down job, and most everybody wants to take the cheapest option available to them, which is nailing. Most standard installs of 3/4 solid strip hardwood should be done with nails.

Your post however was asking about herringbone - herringbone pattern has a lot more seams due to the amount and size of boards, and as such, it is prone to more issues with overwood/underwood, gapping, and noise from loose connections. I will always recommend herringbone be installed with a full trowel glue down for maximum adhesion to the subfloor. You also should not install crooked herringbone boards if you get any; leave them aside for the smaller pieces you'll need at the walls. If a customer wanted me to nail a herringbone floor, I'd make him sign a waiver first.

I don't understand why uses nails at all, ever?

Its a cost thing - glue is more expensive than nails

Same reason why people opt for floating floors over concrete, its much cheaper than a glue down method.

regarding a full glue down---I realize if I wasn't going to nail, at all, then a self leveling compound could be used. But even with a full glue down job wouldn't you want to also nail it, if for no other reason to have the nails/staples act as clamps/hold downs while the glue sets? If so I'm back to my first question as how to flatten the floor in such a way that allows nails to be used.
As this would be done on my own house I'm willing to put more time into it and if that 'time' gets spread out over several weeks, no problem. Level would be nice, but in a 120 year old house I'll accept that it probably won't be level, but flat is something I do want to aim for.

You do not need to use nails to hold the boards; they make flooring straps, you can use boxes of hardwood, I've used pails of sand in the past, etc. Lots of ways to keep the wood solid and tight while the glue cures. I've put plenty of herringbone in condos which are concrete subfloors and you have no option to use any nails whatsoever.

It's your floor - do what you want; if you want to use nails that's your prerogative. I would advise against it as its just not necessary in my experience. I wouldn't let my customers pay all that money for an expensive herringbone floor only have installers put a bunch of face nails at the perimeter or have a gap out mid field because there was a nail misfired. Herringbone is not something you want to be repairing if you can avoid it by doing the install right from the start.

I'm not trying to challenge you, just to better understand-----Obviously in some situations, like where you have concrete floors, I realize nailing isn't a option. But presently that's not my situation, but if I use a cementitious leveler then gluing is the only answer.
But when you are doing a full glue down job herringbone floor over a wood sub-floor are you saying that you still don't use any nails/staples? You say you like to use pieces that are perfect, and who doesn't, but getting a couple of room fulls of 20" to 24" long pieces that are perfect, really perfect without any bowing or slight twist seems to be expecting a lot more then I've experienced. Wouldn't using a couple nails to take out a 1/16" gap and/or to keep the wood flat while the glue dries be a good idea and make the job go quicker and smoother?

Again, if you want to use nails, go right ahead. I'm only saying its very possible to do it without, and that all the herringbone floors I've installed were done with full trowel glue method (in fact, I worked for a shop for years where we didn't use a single nail at all - only floating, T+G glue assist floating, or full spread glue down were our only offerings; we also only sold engineered floors). Herringbone takes a lot of patience and yes ideally the material will all be straight and free of defects. I will also always push my customers who choose herringbone to select one with a bevel on the sides, as I find the straight edge/no bevel version tends to show more of the slight imperfections. I also set aside any not straight pieces to be used at the walls or in closets etc. If you are expecting a lot of bad pieces, or already have your material and its got a lot of bad pieces, contact the manufacturer and send it back if that's an option.

If it makes more sense, I'm only talking about pre-finished herringbone - I know in the old days the sand and stain on site herringbone would have been nailed, but I'm not about that life

I've had good luck leveling out dips in subfloor with Levelquick RS. There is conflicting opinions out there on whether or not it can be used with hardwood but I'm now 2 for 2 success-wise. In both cases the dips were pronounced enough that I used steel mesh for additional firmness and my floor gun had no issue pounding through the Levelquick with the exception of maybe 3-4 staples.

It's not cheap stuff to work with but will get the job done.

In both cases I think I let it dry for about a day before installing the hardwood over it.

I don't think I ever needed to address 1.5" dips though, at its deepest I think mine were a little over an 1".

Would be good to test out first as this stuff is impossible to remove once it's on short of taking a jackhammer to it.

Thanks
Steel mesh---meaning the sheets that are 2'x8' and maybe 3/16" th. and often used for plaster repairs? Does the underlaying ply require a primer or a bonding agent?
And yes, this would be a sand and finish on site job, not a pre-finish job. I hate those factory bevels!

I always start by sanding down or planing the high spots. The low spots aren't so bad, then. I use 16 grit paper and an edger. The B-2 is my favorite.