Question regarding a fair payment for my builder/contractor

[Peter Gray]

My situation- Currently my uncle and I are rehabbing my first fix n flip. I drive an 18 wheeler during the week and my uncle works on the property while I am gone. On the weekends I help out.
Since It's family he won't charge me contractor prices but I want to pay him a fair price.
He isn't employed anywhere else and doesn't necessarily live on what I can give him on a weekly basis knowing that if this first flip works out well we can slowly build this business together.
So, my question is if I offered a percentage of the profit at the time of sale what would be fair?
"The quicker we get this project done and sold, the quicker we make some money" is what I figured to use as a motivator.
Thanks.