Hi Rewker - I just finished helping a builder in California with a load of questions relating to an AKW Shower pan and bypcrete. You need to be very careful here you are getting a tile approved substrate.
You need to worry about the thickness you are getting.
Is it safe to assume you are pouring Gyp-Crete over radiant heating pipe? Do you know you need a good 3/4" over top of the pipe. After a lot of back and forth the builder I was helping aborted the Gyp-Crete all together. I urges him to check with his structural engineer and the engineer told him his home was not designed for that kind of weight.
Think about your stairs. Transitions from tile to hardwood,
There is a lot going on that you might not understand. My blog page has a little more info on the subject. Your first step - call your engineer. Ask him or her if 1.5" of Gyp-Crete is OK. Then use some 2"x8" 's nailed flat over the area your tub will be installed. Pour the Gyp-Crete even with your bottom plate - 1.5" (most builders in Vancouver use a double bottom plate).
Slow down. Do your homework.