I agree with the others... the poll didn't really give a "good" option.
The biggest problems I see with "the rule" is that there tends to be lack of "common sense" for a lot of people...
I had a customer try to tell me that it was ok to put a 10 inch oscar in a 10 gallon tank... yeah.. right... poor fish (it died, of course).
The other problem I find too often is that people forget to count it as 1 inch of ADULT fish... instead they buy a baby fish and don't allow room for growth. A good example is a 30 gallon tank with 6 angelfish... because the angelfish are all only 1 inche each, it is thought to be ok to stock this way... but angelfish top out at 8 inches each, so the minute they start to grow, water quality and aggression levels start to become an issue.
There is no exact rule, as was already stated, but always work with a fish's adult size when figuring what size tank is needed.
Another note is that filtration and water changes say a lot, too, as well as feeding habits, species of fish, etc etc. A lot of good points were made in this thread, I would heed it all.