Informal Coaching Profit Sharing %?

Hey all,

I feel like a total n00b posting here, but I'm trying to work out a coaching deal with a student. While we have done $ per hour before, we would like to work informally on a profit sharing basis. Basically, I will be available for chat/general strat/questions and sometimes sweat sessions, no set hours, and I will take a % of his profit.

The question is: What is a fair %?
http://www.casinosviplounge.com/Casino-Del-Rio.html

I've had a similar situation, and seen others on 2plus2, which included staking for a 50/50 split. So for just the coaching, does something like 25% seem fair? Again, there is no minimum hours of coaching (or him playing for that matter) and we don't want to set these. Any advice? Thanks!