Wow, tough situation, tough question to be asking. There isn't a book somewhere that has this kind of thing written down. Ownership normally starts with shares, like shares of stock, even in small businesses. There are no formulas.
You should probably be talking about this to an attorney.
In a talk at Columbia University, the PayPal co-founder and venture capitalist discussed why startups need to go 'beyond professionalism.' But is that mentality perpetuating Silicon Valley's diversity problem?