Ex-reef tank keepers tend to go way overboard on light - they buy the 200 watt fixture for a 20 gallon tank. Pure beginners do the opposite - they buy the 15 watt fixture for that 20 gallon tank. That is why I still like the watts per gallon standard.

If you shoot for about 2 watts per gallon with any fixture you are at least in the right ball park for having good lighting. Maybe you need to add 50% more light, or turn off a quarter of the light, but you rarely have to junk the fixture and buy another one.

Then I believe there is another ballpark number: T5 bulbs with individual parabolic looking reflectors are 1.3 to 1.5 times as intense as AH Supply type bulbs with AH Supply type reflectors.

And, "Power Saver" screw-in spiral wound fluorescent bulbs are probably less than half as intense as AH Supply type bulbs and AH Supply type reflectors.

I'm not at all sure where T8 or T12 bulbs fit in there.

Keeping these simple ratios in mind I think you can get close to a good light for any 20 gallon or bigger tank.