Hello to everyone. I'm new here, this is my first post, I have a problem and would greatly appreciate some feedback and/or advice.

I'm preparing to install 3/4" solid prefinshed maple planks 4 1/4" wide, onto a 5/8" plywood subfloor. The problem is that this subfloor has a section that gradually drops in elevation starting at a joist, running towards a side wall. I've taken some sample measurements to give you an idea how "severe" this drop is.

Starting at section's left boundry:
1/8" drop over 13" line segment
5/16" drop over 21" line segment
7/8" drop over 30" line segment
1" drop over 31" (lowest point)
1" drop over 32"
7/8" over 31"
3/4" over 33"
3/8" over 34"
Ending at section's Right boundry. The span between left and right boundry is 148"

My tentative plan to correct this, is to lay 5/8" (or thicker) playwood on top of existing subfloor in areas that will accomadate it. Then gradually shim back the valleys using 30# felt roofing paper, and finally level it all out with large amounts of wood putty (a product called "Durham's Water Putty" would work well for this)

What do the pros out there think of this approach. I'm open to questions, comments and most of all suggestions for alternative solutions. TIA.

Are you trying to level the floor or make it flat? Some of those are out of spec but some are also very close. Big difference between flat and level.

Yes, I understand what you mean between flat and level. Flat is good enough for me and yes the outer boundry measurements are very close ... almost a non issue. The middle section part however, is bad. Do I have the right solution approach or do I need to adjust it?

I think it's better to pull the panels and shim under then on the joist. I have the opposite problem right now and I am sanding the heck out of a couple small humps.