Spot on on the 10% idea. I have a crappy little club and only two of us play the credit games I set up. As we only play for 30 credits, the winner ends up feeling it's not worth his while.
Haven't set up a credit tourno for over a month now. Might as well close it down.
Some of us still buy our credits. Thus, getting charged again for using them seems harsh.
Overall, a 10% rake on CC tournos is fair IMHO.