Background
I'm developing a web-application for an active shooter defense training company. They go to organizations all around the country and teach them how to be best prepared for a horrible unexpected event. They want to take their training program online to reach more people.
Problem
I have a friend that is helping me develop this application. We've agreed to a share of income deal. Any income I make, he'll receive 10% of up to 100k per year. I'm getting a contract written up for this, but I'm not sure how to objectively evaluate his work effort.
Concerns
I'm concerned that if there wasn't some objective way to measure his work, he could be paid for little to no work. This application also wouldn't be generating income until around Q4 2019 or Q1 2020. I wouldn't want there to be weeks at a time that he didn't contribute anything to the project and still have the deal legally.
Question
How could I objectively measure his work-done to ensure he has contributed for the share of income contract?