Oh my! You're a funny chick. Ha ha, I'm in my den. You're in your smelly dorm room taking a break from your calculus homework. If it's not asking too much, please answer the original poster's question.
I would set the % at least at 30%. The higher you go, the more motivated you will be and you need to explain that to the potential client(s). I would actually prefer this over a small yearly mgmt fee and a smaller % of profits.
Negotiate for whatever you can. If the profit sharing % will cover your incremental costs of managing the money and leave you with a profit, you have nothing to lose.
Yes, I used that model a few years ago for managed accounts, with a sliding scale perf.fee of 33% decreasing to 20% dependent on a/c size. It's fine if your overheads are insignificant.