Basic physics.
Rounds are about, what ... 6 seconds long, right?
Each second, you fall yoru current speed, plus half the acceleration for thats econd. So (all speeds in feet/second, accelerating under 1 gravity):
1 ... speed 000, accalerate to 032 ... fall 016' ... total fall 016'
2 ... speed 032, accelerate to 064 ... fall 048' ... total fall 064'
3 ... speed 064, accelerate to 096 ... fall 080' ... total fall 144'
4 ... speed 096, accelerate to 128 ... fall 112' ... total fall 256'
5 ... speed 128, accelerate to 160 ... fall 144' ... total fall 400'
6 ... speed 160, accelerate to 192 ... fall 160' ... total fall 560'
Now, somewhere in tehre you reach terminal velocity -- you just won't accelerate any faster. I'm not sure where in there, though.
That would fit the "500' in the first round, 1000' feet every round after that", which DCollins suggests. It's a nice round number, and it's "close enough" to an actual 6-second "rule of thumb" falling rate.
Well, close enough for ME, anyway.
As an aside -- Feather Fall gives you roughly a 10-feet-per-second falling rate. Swift, but not inuriously so. About a slow jog, really (figure it'd manage a 50-yard / 50-meter dash in about ... 15 seconds).