From my POV, I waited until I was sure that the designs were what I was going to go forward with (this took about 4-5 weeks, maybe 2-3 iterations of different pages and colors). Then I proposed a % of revenue for x amount of time and it was accepted.

The project has moved slower than if I'd hired a designer (for example Lindsey is currently defending her thesis at school and is 100% focused on that). But I maybe always had the trump card by being like "We need to launch before I have a baby".

Overall I think I got lucky and I'm not sure what advice I'd give. Forrst is like HN for (hungry) designers though.