Working on planes, I imagine your into major precision . My father always used to tell me,"We're not building pianos, get it done !"

Usually installers will use some type of 10' straight edge, ( a chalk line will work ), they check to see if the substrate is within the spec. If it's within spec they proceed to install, if not they will do whatever necessary prep, to flatten it out. Flatter is better , but as long as your within "spec", your good to go.

On your felt application.... You will want to make sure the edge of the felt is feathered, ( so you dont have a sharp edge ). If you have a sharp edge, it will wear on the plank seams , at that point. You can sand a feather , but not preferred because the sand paper will gum up. Typically you would use a patching compound,( just make sure it's totally dry before installation). Depending on the area your trying to build up, sometimes you can get away with "stepping" the felt.

Can i cut it at an angle? I wasnt aware that i had to do this, so it adds a whole nothing bit to prep i wasnt expecting.. what type of patch compound would i use?

How thick, or high , is the edge ?

sofar its only one layer of #30 felt layed. so I guess like 1 mil or something not very sure honestly.

IMO no sweat. Ya, just take a utility knife and taper the top edge.

thanks dude.

oh.. wait a sec. I just thought of something.. Im using floor muffler underlayment under the flooring and above the subfloor/felt. do i still need to taper?

Sorry i missed this part earlier. Exactness is preferred when doing a task for me, but im not a machinist where i need to figure out exact tolerances. Generally speaking, I need to find a 3/8 wrench to fit on my 3/8 bolt/nut.

IMO, and others on this Forum, Floor Muffler is the best underlayment around.
Most likely not necessary to taper that top felt edge. But if I was installing the job , for my own piece of mind, I would do it, takes no time at all.

Geez, I think your selling yourself short. I can grab the right size wrench...But building a plane ? Years back I was heavy into 1/4 scale RC, it was a blast. Was a member of the Black Sheep Squadron

Lol, well i havent built one yet.. but maybe some day.

btw thanks a bunch for your Help. I think im getting pretty close on my floor prep. Believe it or not i think my floor was pretty close flat wise. Had a bunch of screw i needed to put in deeper, but other than that, ive found like one high spot and it seems someone spilled some liquid there and the wood swelled up a bit. A few low spots on the edge, but nothing one layer of felt paper cant take care of...

I never messed around with RC's except for one of those cox planes you had on a string and walk in a circle with.

ok one more question. most of my floor is pretty flat. Except i noticed a severe dip up where my sliding glass door is. I tried to add roofers felt and im up to three layers now. but it seems it moves the dip further back as i add roofers felt. In other words the roofers felt is making the actual flat part of the floor dip up.. I can add pics if you cant visualize what im talking about.

Their definition of flat...3/16 in ten feet mean no deviations greater than that (up or down) in a ten foot radius, not just in one direction. Or you can go 1/8 in 6ft. I likem flat as possible.

ok i think im at the point were i need to use this patching compound to feather the edges of my felt. What kind of patching compound are you refering to? like the same stuff for drywall? the felt seems to just keep pushing my gap farther back the more i add. so i guess that is due to the same thickness of the felt.

Felt has limited applications, it is not the way to go, with what your trying to do. A few patching compounds that installers use are, Ardex, Durock, Vitex.