Iron body multiplies weight by 10. That'd be my guess for green star adept as well. What's the reasoning behind 8?
There are actual rules
Yes, but the rules for falling objects say that the max damage is 20d6, period. So, by that logic, I can only ever deal 20d6 damage to falling on stuff from at least 10 ft. I used physics to extend this concept to include the scenario of intentionally using yourself as a kinetic weapon. Falling assumes you're trying to slow your fall and/or not intentionally using yourself as a kinetic weapon. Nor does it cover scenarios at terminal velocity, where having a maximum damage of 20d6 really isn't intuitive. That says a human who fell 200 ft (P of 6,300 kgm/s, assuming a 90kg human) does the same damage as the GSA at terminal V (267,300 kgm/s).
Bull rush doesn't cover scenarios when the bull rusher is moving at terminal velocity, and neither does sunder or other combat maneuvers.
Also, falling is not an accurate way of describing the idea. I would say its something more of, "character ballistics," which doesn't have a section in the rules, and where I decided physics would help. Yes, It's closest to falling, but it's not.
I remember seeing 8x weight somewhere the last time I played (2+ years ago), but I can't find it and I like 10x better.
So, using the 10x,
terminal velocity in bullet position: 297 m/s
new P at terminal V: 267,300 kgm/s
new damage at terminal V:
303d6