You can cut diagonals as your contractor recommends. It's not necessary though to achieve a slope, which you do need for the reasons stated. A very minimal slope is all you really need. Either way, however you cut the slab, you should not see the seams in the slab. If finished out right, they will polish and buff those seams so they aren't visible. I hope that helped. Good luck with your project. .

Thanks Tonianne! This is good information.

Torianne, thank you for taking the time to answer everyone's questions! I also have a question about the waterproofing of this installation. I am designing a similar tub shower combo (with a window!) for a client who wants a large deck around the tub. On plumbing forums I have read all sorts of warnings about how this is a recipe for water intrusion disaster when a shower is involved. I'm assuming you used Kerdi or similar on the walls; how does that waterproofing tie into the tub itself so that shower water drains properly to the tub? is the deck below the slab waterproofed to the tub edge, or does the waterproofing land on the top of the slab and rely on the caulked joint between slab and tub to keep water out? And it sounds like you had the quartz fabricator include a slope to the slab, can you explain that? Is it 4 pieces canted and joined, or is it all one slab that has been ground down for the slope? I REALLY appreciate your help. The general contractor is very wary of this idea!

Stacey, How did you make the connection from the wall water-tight? Thanks .