For me, I would say > 20% would be a "co-founder" status. Anything less and it sounds like you are more an employee than anything else. It may be his idea, he

For me, I would say > 20% would be a "co-founder" status. Anything less and it sounds like you are more an employee than anything else. It may be his idea, he may have even created the prototype, but a prototype is not a final product. If you have to trash the prototype and work from scratch, you'll be putting a lot of work into something that is barely yours. I wouldn't, however, ask for >30%. If he's gotten into an accelerator and programmed a prototype, he's probably been working on the project for quite some time (a year or more). To ask for almost half of the equity would probably be too much.