There is no rubric for you to follow. Negotiate with them what you think you are worth as far as value to the growth of the bottom line, make sure to also consider possible upsides for the company you will be joining not being greedy for stock alone, consider shorter vesting period.

How many more employees will be joining, what does the team plan on doing with the rest of the equity... how much funding will it be needing in the near future.

All these can be argued as "not your problem", but when you join should consider things as a team.