> I worked extensively over the last 5 months and contributed significantly to major milestones, through which we gained significant traction with client/ VC interest.
Key word: interest.
In other words, you're still pre-revenue and in fact don't even yet have an MVP. And the company is operating on bank of mom.
This is a very, very high risk position to be in. You should ask for 6% and do not take less than 4% plus an executive title. It's not just a title for title's sake, you need to have a seat at the table.
Do your best to negotiate in the abstract, ie don't let familiarity or even friendship with the founders muddy the waters. Use your leverage as "extensive" contributor up to this point. Once you start this negotiation you have to set a time limit (2 weeks?) to complete it and be nice about it but be clear you will not be making further contribution under the old terms. It's 6% or nothing, not 6% or continue for a while longer as you have been doing.
There's no math behind the number. Any math that is out there is just rationalization.