You've got to use a mixture of things- your own judgement, other people judgement, and the other rules but with a pinch of salt. Increased surface area may allow you to have greater stocking as it increases the amount of oxygen in the water (though this isn't the most important reason surface area is important as it is quite easy to keep O2 levels up). Plants(if you've got a lot of them) can also let you have a slightly increased stocking list as they can help remove nitrates and ammonia etc. from the water, they also help with incresing oxygen levels in the water during the day, however they are a drain on oxygen level during the night. Also the one inch/gallon rule only applies to small slim fish- you couldn't have a 10 inch fish an a 10 gallon tank but also you couldn't count a 1 inch slim fish and a 1 inch deep bodied fish as taking up the same amount of your stocking allowance, the same goes for fish that produce a lot of waste (like Plecos or goldfish). Also the activity of the fish needs to be taken into account- a very active fish, such as a danio, will need a lot more room than a less active fish of the same size- you've probably seem that people recomend 3/4ft as the minimum tank size for a danio yet they're quite small fish. Also look at agression of the fish- an agressive fish is going to want a much larger piece of the tank than a peaceful fish (unless it's a rift tank where differant rules tend to apply). On the same note shoaling fish won't mind being in slightly closer quarters than solitary fish- though they still need enough space for all of them, it's almost possible to view them as one very large fish.
Then you think how fish actually swim, horizontally not vertically, and you can see that an increase surface area may let you have more stocking than a taller tank because the fish will have more room- they don't particularly care what's above or below them, only whats infront or behind them.
And yes area is more inportant when taking into account strickly top or bottom dwellers as they don't have any room at all to be swimming up or down.
So i guess you need to take both the inch/gallon rule and the surface area rule into account but niether should be relied on too heavily. Yes you could put more fish in the 620T but probably not as many as you could in a differant 29G that had more surface area.
Sorry for being to wordy but from the research I did when i first started deciding my stocking levels this was what i found but I'm sure someone with more experience could give you some more information- and I'm sorry i didn't just answer your question i tend to get a bit too keen
