Hi Laura! We are in the middle of a similar tub/shower remodel and have also chosen to frame an under-mount tub we are topping with a marble deck. Our contractor recommends we have diagonals cut at the corners of the deck, so the four sides can be individually tilted to divert shower water back into the tub. Have you done something similar here? It's hard to tell. I am not sure how conventional his idea is and how the overall look will be effected. What are your thoughts?