I would work out the difference between the salary at say Google vs what you took during your tenure at the startup then double it because of lack of upside.
Aside: If the co-founder is an HN reader, then they probably know the throwaway account is you. That is going to skew this negotiation.
Aside: If the co-founder is an HN reader, then they probably know the throwaway account is you. That is going to skew this negotiation.