Help
Mark,
You're in the right forum to ask for help. I think that the question is: what is the wood. Is the 20% the wall thickness or the moisture content?
If the wood is green, you can finish turn it to a wall thickness of 1/4", with consistent thickness throughout. Then it will probably warp somewhat.
If you want to rough turn and let dry, turn to 10% wall thickness of diameter (i.e. 10"D = 1" thickness) and set aside for 3-6 months. Return the outer, and finish turn the inside.