How to calculate BTU for window AC?

My partner and I live in a 100yo, unairconditioned apartment in NE Ohio, and am starting to die as we get farther into summer, but I'm struggling to figure out what AC to buy. I keep getting stuck a what size AC, and I've tried to run calculations and come up with wildly different numbers very time. 18,000-24,000 BTU feels like overkill for my 350 square foot living/dining/kitchen area, and I don't even think that they make window units that big.

The whole apartment is 650 square feet, with the living/dining/kitchen area being about 350 and the bedroom/bathroom area being about 210. 8 foot ceilings. A normal amount of windows? but the windows are very old, single pane, and a little drafty. I don't know how good the insulation was, but I was overheating all winter, which either speaks to good insulation or overzealous radiators. I have southwest facing windows in the bedroom and it gets pretty warm in there.

If there's any other information I can provide, I'm happy to, but I don't know what else I need to know to be able to figure this out.