Let's say I want to start a project that includes web, android and ios app. However since I'm a web dev I cannot do mobile apps myself. So I thought I could put the code on github so that anyone can contribute if they would want it to. Also, in order to motivate people to contribute, I was thinking it would be a good idea to give 49% of the profit that this project / website would make.

The main question is this. How would I know how much a contributor deserved out of that 49%? Is there a way to somehow measure that? Can you even measure it? Maybe github activity, like no. of pull requests?