Finally got the tub in (Kohler Underscore) - limited by space available, so is 30"x60" ...can be undermounted or drop in or perhaps even push against the outside wall.
Wanted to wait till I laid eyes on it to make sure, but it is VERY straight on sides and ends ... and there is about 3" horizontal space (lip) between outside edge and where it slopes away into the tub itself. I would like to build the platform hard against the outside wall (under a window) to minimize the intrusion into the walk area in front of the tub ... room is long and narrow.
I've seen the tub pushed against the cement board covering the outside wall studs - this would mean only about 3/8-1/2 inch of tile and thinset overlap onto the lip area.
Could I also slide the tub directly against the studs (supported by usual 2x platform topped with plywood which is attached to both floor and outside studs), then CBU down to 1/8" or so above the lip, then thinset and tile down to 1/16"-1/8" above tub then caulk last bit? Would lose another 1/2" of the lip, but doesn't seem like would affect much else - except the look would be more of alcove than drop in. And, would buy back an additional 1/2".