I've read about the "one sq inch per 2,000 BTU's" and when I did the math I thought "That's a big hole!". Then someplace else I read something like " Every building leaks some, figure one sq inch after 80,000 BTU's" so if you have a 100,000 BTU unit, figure the one sq" on 20,000....for ten sq inches. A 4" dryer vent is 12.5 sq inches. That makes sense and is an easy place to start. I've personally seen a few oil boilers that didn't get enough make up air. Every one was cured with a 4" vent.