So, the science here isn't at all "science" in any way you care to think of science. They're guesses based off of statistical data. Your smallest bubbles ascend at approximately 60fpm. When you're deeper, this is more okay as the gradient isn't as severe. Basically, the change in pressure per foot is the same....but the PERCENT change is less the deeper you go.
This might be too much, but I'll try to make it simple in the Beginners section:
Different parts of your body absorb nitrogen at different rates. These parts are called "compartments" and represent different types of tissues. Some are fast, and they absorb and exhaust nitrogen quickly. You also have slow compartments, which do so much slower. Getting bent is (basically) from there being too much of a difference between absorbed nitrogen and the nitrogen you'd have at a shallower depth (or the surface). So, if you ascend too quickly your fast compartments will cause you to get bent. If you ascend too slowly, your slow compartments will keep absorbing nitrogen and could cause you to get bent.
Long, complicated story made short and simple: 60fpm
was the most cited figure. 30fpm is becoming cited more frequently, especially in Tech diving. My dive computer "limits" me to 60fpm below 30ft and 30fpm shallower than 30ft. This is what I try to follow. The reason nobody can give you a GOOD answer is because the only thing we know about dive physiology is that we know squat. I wrote up a (I think) cool article on this using other people's analogies. I'll post a link if I can find it.
Edit:
Here it is. It might be a little complicated, but the point is nobody knows....but we THINK we understand some of it.
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