Area first, then tiles
Tiling starts with the area to cover and the size of a single tile. Dividing one by the other gives the raw number of tiles, and the waste allowance turns that into the amount you should actually buy. Working in consistent units, with tile sizes converted from inches to feet, keeps the result accurate.
Why buy a little extra
Cuts at edges and around fixtures, the occasional cracked tile, and future repairs all eat into your supply. Ordering full boxes from a single batch protects against color variation between dye lots, so a small surplus now can save a frustrating mismatch later.