You won't get something concrete, and if you do, it'll just be whatever you personally find adequate.
The inch/gallon rule is just a basic guideline that people have all generally agreed upon at one point in time, it has its flaws, but it's not a bad start for someone who has absolutely no idea what they are doing. Anything more than that is probably overcomplicated.
The other thing to take into account is that some fish are different. People often say that certain fish, like oscars, for example, are messy eaters, fish like plecos poop alot, etc, so they factor in more for that. As to how much merit that actually has, or what it's effects are, who knows. I do know a fish like a goldfish who has no stomach is constantly grazing so creates a lot of waste for it's size, but other fish like O's, Mollies, etc, I've not seen any difference in their waste/messiness versus other cichlids and livebearers the same size. I guess some fish just get a reputation.