I have nixed the Papercrete idea in favor of silicone, After realizing the bathtub was out of level only about a 1/8th of a inch , I opted for another solution -use Silicone to bond the Tub in place , its shimmed on the center floor joist of course
I Put a whack of latex silicone on the tubs base, sealing the foam that I exposed when leveling it and filling the imperfections in preparation for being mounted.
I ended up using 10 cans of spray foam so far , the 11th can will go on after its mounted for some finishing touches when mounting it.