If all you want to do is level the floor directly under the tub then that should do it cause it doesn't matter as long as the tub is level. If you are leveling any part for floor tile/vinyl/carpet then use the leveling compound although if you are ceramic tiling the floor then thinset mortar used for installing the tile cause it can be adjusted a little in thickness to compensate for minor leveling problems.
Since it appears we're both online now (I've only got a few more minutes though) does your tub have any base rails? If so then shim them up to level after installing your double layer of plastic. Then just scoop some mortar in from the end if you can. Otherwise mix up a batch of mortar and pour it on the plastic, then plastic on top, and then set your tub in. I assume you have a rail around the 3 edges if this sets in a tub space. Try to keep the tub so the weight is supported by the base rails/mortar base and not the side rails. You have to guesstimate the amount you need by the height the tub is off the floor. Shoot for the tub side edges to just barely touching or about an 1/8" above the side rails. Also keep the drain area clear of mortar.
To answer your question, the mortar thickness may be minimal close to the drain up to a few inches or more in the back depending upon how high your tub is setting off the subfloor. Remember the tubs slope to the drain.