Actually 1 inch per 5 gallons makes sense.
I know I read the other ratio somewhere. It wasn't for freshwater fish but it may have been old, outdated info. (I tend to over-research stuff and with that, I do come across practices that are no longer used).
So I guess I need to be content with my 4 little fish (approx. 10 inches) in my 55 gallon.
And I love Yellow Tangs too. We want to upgrade our tank size just to have one.