I'm with Michele, once you go flat fee, I would NEVER go back to percentage. It's easy to say you might have lost a client due to that but there is probably more to it, you are in business like anyone else and deserve to get paid for your time. This stuff about "getting paid when they get paid" really is becoming a thing of the past, in today's economy you have to change along with it, if you are good at what you do there shouldn't be any questions as to HOW you are charging, make sure your pricing is fair and up front.