I'll go from experience and that's about all. My friend has a bagged obs that was bagged on 22x9.5 +10 on a 255/30/22 tire which is 28" in diameter.
That being said, running a 20x10 you'll need AT LEAST a +23 offset to sit in the same spot as him running a 28" tall tire, which in a 20 on a 10 wide rim would set you with about a 295 35 20 which measures to 28.1" tall and just a hair over 11.5 wide. With it being 1.5" wider over all you will need to compensate with offset. So over all to tuck a 20x10 with a 295 35 20, you'll be looking at an overall offset of +43 or better.
So you'll be looking at 7.2" of backspace or better.
If I'm wrong, Brett will correct me.
Here's pics of the obs I mentioned.