Nope. I just go with what seems right for what I'm doing. I try not to stuff the conduit, but it has happened before.
You can do what you want, but if it were me, I'd run the lights the other direction with them butted up end to end, especially if the ceiling is ribbed metal (the ribs will run the long way and you can run the lights between them). At 40' deep (outside dimension), you're closer to 39 on the inside so four 8' fixtures would be about right. You can probably get by with two rows, but I'd have three if it were up to me; one down the middle and the outside rows about 3-4' off the sidewalls. You'd wind up with either two more or two less fixtures, but would eliminate a ton of conduit on the ceiling since you'd only have to pipe to the end fixture of each row then pull the wire through all of the lights. With the rows being 32' long, you could also add a 4' fixture to each row but that's probably overkill.
