How wide is the deck of your undermount tub? What material did you use?
From 4" at narrowest to 6 3/4" at widest. I think it is a good idea to keep your tub deck narrow enough so that you can easily get in and out of your tub. We put in a slightly curved tub deck because it needed to be pretty narrow near the door opening, arching wider toward the middle of the tub. Our tub sits between two wall openings, with lots of space in front of the tub. So the deck starts out straight on both sides for a few inches, and then curves out to about 6 3/4" max. This has worked great for us because we can stand closer to the narrower part to enter the tub, but can put soap and stuff on the wider part. Before we remodeled our tub deck had a step, which made it difficult to get in and out of the tub. I would highly recommend never doing a step.