It's definitely not a rule of thumb. The best calculator for port area was put up and made by a member on here, however I won't use the site anymore as it has numerous warnings from Chrome and my Anti-Virus/Malware/Spyware scanners telling me the site is a problem. The one on Carstereo.com is better than nothing or if you're not shy about doing some math the actual T/S formulas are out there. I tried doing a search and couldn't find anything but simple calculators, not nearly as accurate at the one that I can't use anymore, lol. EDIT: Found the actual formula's HERE. The formula's calculate minimum diameter in centimeters. A little more math and you can convert it to inches, then figure the cross sectional area to know what size the slot needs to be. Again, got to not be shy about the math, lol.