Stock contracts aside, the best you can do is what is fair.
Here’s how I might think about it:
- $50,000 invested at say $1m valuation = 5%
- 10 year commitment as founders; he did 2 years of it. So vis-a-vis the 2 of you, she/he might get 20% x 40% (if your equity ratios are 40 you:40 her/him: 20 others), or 8%.
- So 5% + 8% = 13%.
That’s a lot. It may be too much. If it feels unfair, re-evaluate. It also assumes no other funding. That funding would dilute the maths substantially (and might lead to a number that seems more fair).
And you’ll certainly want some sort of proxy / voting agreement where they have to vote with you on funding, M&A, etc. if they own > 5%.
But the math seems about right.