the 1" per fish rule is VERY VERY limited in it's application. it pretty much only works for when you have a larger tank that you are trying to ESTIMATE the bioload/swimming space for most AVERAGE sized fish. even then there are exceptions, like
goldfish.
even for a full grown angel, a 5 gallon tank wouldn't be big enough. so you are right, the 1" per gallon rule isn't really applicable to large fish, or small tanks.
i wish i could find it, but somewhere someone suggested using something like you said and figuring it cubically as opposed to just a length measurement.
again, it's just a loose guideline and if you have a pretty good feel for your aquarium and test enough, you can find your proper stocking level. if your fish are too territorial, you might not have enough space. or if your nitrates are high all the time, you may be overstocked.