Interesting way could be using the approach suggested in the book "Slicing pie" by Mike Moyer

You can find in on Amazon: https://www.amazon.com/Slicing-Pie-Company-Without-Funds-ebo...

Funny you ask this here. In 2012, 1 in 4 Y combinator starts up broke apart due to founder disagreement. You can bet a large part of that was due to equity distribution issues. Equity issues can get really messy - and easily damage friendships.

Read the reviews for Slicing Pie. It's about maintaining a dynamic model that eventually vest into shares (if the company works out). I read it. It's great quick read. Ideal for your situation, I think. And it may preserve your friendship.

http://www.amazon.com/Slicing-Pie-Company-Without-Funds-eboo...

Interesting, though it's just a short pitch for a not-free book: http://www.amazon.com/Slicing-Pie-Company-Without-Funds-eboo...