"...but if 3,413 BTU + outside ambient temps is all I need to cover then I think I'm in good hands."

While OM's formula (LOL! Is this like Ohm's Law?) is good for compensating for the lights, (3413BTU/H there is always a time factor guys!) there is a lot more to doing a heat load calculation on a room, any room, and especially a grow room...10,000BTU/H should work for you unless you've got some kind of extreme conditions outside. If the A/C runs constantly trying to hold setpoint, or ices up, tho, you will know it's too small for the conditions you have. If it does not run long enough to control the humidity to at least 50%, but reaches setpoint easily, it's too big. (By too big or small I am referring to the BTU/H rating {yes, it's just a rating, it may or may not achieve that number}number, not physical size, sorry, but you'd be surprised how many think that's how you measure an A/C!) If you find a need to rig another booster fan that runs when the A/C does, let me know and I'll draw you up the circuit...it's easy, you just need a fan relay in line.