I have to think that putting it into a formula is difficult. It all depends on risk, obviously: how certain is a person that the company will be worth something? On top of that, you have ability to get by at a given salary. If you have enough saved to live for 5 years with $1/yr salary, then you can afford more of a risk than if you can live for 6 months without a salary. Or if you can afford to live at half your salary, but without saving for retirement, where does that leave you?

It entirely depends on the person and how risk-averse (or not) they are.