If you have the room you should leave some space for a second enlarger. You can use this for color, flashing of paper, or just setup for a different format. Your 5x3 space should be OK for this, but I would rather have a bit more room to keep paper boxes out, notes, etc...

I would also put in more than one breaker into the darkroom. I would put the lights on a separate circuit from the rest, so if you trip the main one you still have lights. Also having a separate circiut for a paper dryer, hair dryer or microwave would not be a bad plan. And your power is more likely 110V, so a 100watt bulb is actuall drawing about .9 Amps. But your math makes for a safer load calculation.