Redoing a bathroom, and the floor is all over the place (three different heights).  The subfloor in the main walking area is 3-1/4 inches lower than it needs to be, while one area is 2-1/2 inches lower, etc. (some joists were cut down 1-1/4″ from others in the original framing). 

The finished floor height needs to be 3-1/4 higher than it is to match the hallway entry.

As a result, I need to build up one part of the floor by 2-1/2 inches, another by 1″, and the third area by 1-1/4″.  (True, precise measurements–plus 3/4″ of mortar/decoupling membrane and mortar.)

Since plywood actual measurements never add up to those dimension, how would you suggest I layer the plywood subfloor to get as close as possible to those buildup heights?

For the 2-1/2″ depth can I just lay a 2×4 across the existing subfloor (in the area that has it!) and layer on top of that?  Can I use tar paper as “shims”–or is there a better material to sneak up on those true heights/depths?