Why would you not just say 10-20 hours then? If you know that you're going to wear any overrun costs, you owe it to yourself to give the estimate based on the worst case, NOT the average case.

Edit: If you think that 20 is a real outlier, and 12 is your average case, use something like the PERT formula: ( [Optimistic] + [4* Realistic] + [Pessimistic] ) / 6

If O=10, R=12 and P=20, that would give you an estimate of 13 hours.