Porcelain tile

I've been working 3 days now on the new floor. Put down 3/4" cement board. I've done 2 baths and 1 kitchen, but this is the first time I put mastic on the backside of the cement board. I don't know if this will make a difference. It was recommended by a contractor (although, I think I should have used masonry mix). The cement board is heavy! Took 2 days to get it down. I have 2 entry ways into the kitchen, so the tile had to be laid so both entryways had a full tile going in. Using 18 X 18 porcelian tile. Cut and laid the first 2 rows to run up to the entryways. The next day laid all the full pieces. Will let those dry over night and then go back and mark and cut the edge pieces. This takes an extra day, but I find that it makes life easier and it is less hectic and stressful. Concerned about the 18" tile that ends up on the seams of the 3' X 5' cement boards. I hope the grout will not crack...