Took the stuff back to HF, scribed the seat and took it for trimming. That was good advice, thanks for steering me in that direction. Here's the fit, pretty good for walls that aren't very straight.
I thin-setted it in place yesterday, (back-buttered as advised), and this morning it's as though the portion of the slab that has the thin-set under it appears darker. Is this normal, and will it eventually change back to how it was?
