After I calculate the actual square footage like Legacy Industrial did for you, I always add another 5% to 10% to my calculations so that I have extra pieces
( if using tile) or additional paint or epoxy etc for "surprises" that always seem to come up on my projects.
Also with tile I like having a few spare pieces in my shop so that if for some reason I need to replace a tile down the road and I like knowing it will match because it is from the same lot.