Questions & Answers

I am trying to do the vertical layout of the shower walls using 2-1/8" x 8-5/8" porcelain subway tiles, set horizontally with 1/16" grout gaps. The starting wall has a window, and I want an even number of courses to end up butting to the bottom of a Schluter metal trim strip that finishes the intersection between wall and sill. There is also a niche with the bottom shelf horizontal to the window sill. 19 courses @ 2-3/16" per course (2-1/8" tile + 1/16" spacer) would give me a horizontal line where I could put a starter board and go up. I would have a space below that I could scribe in the bottom course without tile slivers. My question: Am I cutting it too close with the math? Is it legitimate to expect to end up right where I want to with strict math multiples [2-1/8" tile plus 1/16" horseshoe spacer for grout]? Are there unknowns that would make it advisable to lower the temporary starting board a shade then sneak up to the finish line as I approach the trim strip?