Sounds pretty good, use morter not drywall mud under the tub and give yourself another inch of space between the bottom of the tub and the sub floor so the morter will fill that area, make sure you have a good amount of mud under the motor so it does not vibrate. Make some shims that are the same thickness as your future tile and cementboard and put them on the plywood deck so the tub will rest on them and be level. be sure to use plenty of cement under the tub so it suported by the mud and not the deck. put something heavy in the tub (another bag of dry morter)after to hold it down for 24 hours. Good Luck.