I month ago I wrote a fairly popular article about startup equity distribution among co-founders. It generated a nice little conversation. The short answer was there is no short answer. Dividing startup equity is a complex process. Doing it right takes a lot of time and effort.
Yesterday I stumbled upon a post by Frank Demmler where he talks about the Founders’ Pie Calculator that he "invented." While I don't think that the calculator provides an absolute answer (and neither does Frank). I do think it is a useful tool that could be used as you think through how you should divide up the stock equity in your startup.