Math.

I get the math part :) But tiles will have to be cut. Where best to place the cut tiles?

at the ends and really it is math to get it right start in the middle with figuring layout and then adjust until you have equal pieces at the ends as close as possible There are some youtubes to help with tile layout . The trick is to put whole tiles where your eye goes and the cut ones as hidden as possible

Your tile mechanic will cut it and place it where it is the least conspicuous. There are a lot of ways to get a ''simple'' tile job really wrong. Be sure to check references, website, Facebook age, and license and insurance. Lot's of cheapo $5 a square hacks out there. A quality tile splash should be 1-5K.