About 28...31% is the long-term average outdoors, provided you don't overlap, double-bomb or other things like those.
It might well be not static in time (although it's been this way for about three years) and not the same for all planets, but the differences are likely too small to detect reliably.
Short-term volatility is quite high - on a 50 drop run, you may as well get 15% as 45%.