Ask HN: Renumeration working part time with a start up

I'm talking with a friend about helping him take a technology he is licensing from a university and productizing it. In essence, I would be the product manager for this product/company. The technology is very interesting and about 70% of the way to "shipping quality."

My job would be pretty standard product management: Determine the feature set needed to ship, figure out how to position the product, ensure launch readiness, and probably roll over into marketing as we go public.

While I'm sure I could put in as much time as I wanted, it will likely be around 15 hours per week until we get the company to a point that its either making money or is ready for seed funding.

The question is what should I ask for in return? If I went straight-percentage of the company, what is a reasonable percentage? Would there be any reason to ask for cash (discounted rate) and stock?

Thanks much!