I've written two books on this topic that described a method for determining exactly how much equity each person deserves. The model, called Slicing Pie, has two parts. The allocation framework determines how much each person deserves. And, the recovery framework tells you the right buyout price (if any) when someone leaves the company.
The model into account the relative value of the various contributions of time, money, ideas, relationships and any other resources. It is dynamic meaning that it changes over time to stay perfectly fair.
Every other equity model relies on rules of thumb or guesses about the future. Because traditional models are always wrong, they are often paired with vesting schedules which are attempts to mitigate the damage caused by mistakes.
The Slicing Pie model is the most reliable way to make sure each person gets the equity they deserve- no more and no less.
If you would like to take a closer look at how it works I will send you a copy of my book. Just contact me through SlicingPie.com