i have heard the rule 3500 btus per 1000w light. but i like to go with 5000 btu per 1000w. the reason for this is because you dont want your ac just running ot. you actually want it to cycle on and off like it is designed to do, which also saves big in energy costs over the long run. straining an ac unit only costs you more money in the end.
Keep in mind BTU's can be subjective. Portable units don't even come close to their rating. A 12,000BTU ductless mini split is more powerfule than 3-10,000BTU portables. I know this because my last grow I started with portables and moved to the split system. I agree with footbal that the A/C will last longer if it's sized big enough to cycle, and not need to run consistantly. I've only had one grow with all bare bulb lighting. Most of my ops have been air cooled lights...and with good reason. But, you don't have that option. My bare bulb grow was 4 - 1000 watt lights in Adjustawings (fantastic reflectors). I had an 18,000btu Fujitsu Mini Split in a 100% sealed room. The A/C unit MORE than handled the lights and a CO2 burner. In fact, I once wanted to see how cold I could get the room with lights on. Running full time, the A/C got the room down to 71F. Not too bad. At 78F the A/C was probably running around an 80% duty cycle.
When growing purple strains, you can crank down the nightime temps to 60F and really bring out the color. You'll also find that the split units do a great job at removing humidity. I've never needed a dehumidifier with a mini split A/C. The room was a constant 78F and 40% - 50% RH.
I think your 30,000 BTU unit will come really close to doing the job. If you have to, just lose a light. You'd be better off with 7 lights and perfect temps, than 8 lights and a hot room. I can't help but thinking there
must be a way to get a couple 8" ducts routed to the outside. Keep brainstorming it. If, in the future, you could air cool your lights, the A/C will be running way less and save you some $$$ in power. Probably enough to cover whatever construction costs are needed to get the ducts in.
In the meantime, take some pics of your build. For 20 years I've dreamed of having a basement grow op. Here in California there are no basements, though. So I became the 2 car garage grow op building guru...6 of em! The most lights in bloom I ever ran was 4. That's why we moved up to a warehouse. I still dream of the ultimate basement op. Warehouses are a pain in the ass.