I wrote up a model for dividing equity in the very beginning, and it often ends up with different splits among co-founders.

See: https://payne.org/blog/dividing-founder-equity-in-the-very-b...

TL;DR: (a) figure out ratios of ownership in the beginning (vs absolute percentages), and (b) use simple "buckets" of contribution to figure out the ratios should be.

For example, the two founders that have been working on the idea for 6-12 months may end up with more equity than the next "founder". Or a part-time (e.g. academic) co-founder may get a little less.