Ask HN: How do I value a potential opportunity to co-found a startup?

I have an opportunity to co-found a startup that has a principal founder and a team of 11 other sweat equity partners/founders. I would potentially bring engineering talent to develop the software (the entire product is a software platform) and at the moment they have about 50% of that software finished (some other members of the team of founders contributed that).

the principal founder holds 51% of equity. the other 49% equity is pooled between the entire team of other founders based on their time/effort. I have an opportunity to negotiate a different deal. A fixed percentage of equity in return for a fixed commitment of time/effort.

My problem, is that I cannot really quantify any of this and how much equity I should ask for. Are there any similar stories, articles, or materials to help me learn more about this type of startups and how to negotiate a good deal?

even better, how to value this startup, that has not yet launched any real product but I personally believe they have good product idea and potential?