Assuming your plan is to level the entire floor area i feel that using tapered shims with the 3 4 ply would be the best of the options presented.
Best for leveling floor for bathtub.
Again it s rechecked for level on all sides.
I have placed perfectly level lip support this is alcove install and the tub sits nicely on the lip supports and it s level but it s between 1 5 and 3 8 above the floor.
The way i see it i have two options.
It s off about 3 8 over the total 5 run with the low side being where the drain is going.
The best remedy is replacing damaged subfloor panels with new ones.
A subfloor is installed on top of a structure s construction floor and provides a flat level surface for finish flooring such as tile or linoleum.
Additional shims are added as necessary to level the tub.
You will have to deal with the extra floor height.
Often the subfloor consists of plywood that is nailed or screwed to the construction floor.
Leveling with shims means setting the tub in place and checking for level then with a helper sliding hardwood or metal shims under the tub where the base reaches the floor.
I am in exact same situation as you were in can t level the floor with self leveller can t add more height and the floor is not level.
Repeat with the remaining three sides of the tub.
The two types of subfloor material oriented strand board osb and plywood are both constructed of wood fibers combined with.
1 install the wall stringer level then shim the outside of the tub under the skirt so it sits level or 2 install the stringer parallel to the floor and let the tub slope forward 3 8.
Their instructions assume an exactly flat floor with no suggestions on how to level but as i said in my first post they suggest setting the tub on the floor with adhesive or mortar.
Make nine piles three on each side of the outline and three in the middle.
Lay a level on the side of the tub and push down on one side if necessary to make the tub level.
When leveled the inner leg sits on the floor and the tub flange sits on the ledger but the outer side of the tub lifts off the floor about 1 4 1 2.