Being a general contractor for some years, I have done a number of shower and tub ceramic tile jobs. I have a faint idea about what you are doing, but the last sentence is a bit of a puzzle. Is there a question that I can answer? Are you planning to tile the walls down to the new tub, or is this a tiled deck that has to meet the edge of the new tub? That suggestion of filling it with water, is a new one for me, I've never had to do that in any of the dozens that I have installed. If it is installed correctly, there should be absolutely no settlement when full or empty. Just my 2 cents. Please ask, and perhaps I can answer your questions.

Roly, the li'l ol' So Cal woody guy