W = opportunity cost value of other people's labor (i.e. if their time is worth $20/hour and they will have to put in 1,000 hours to earn their share(s), W = $20,000)
X = value of the idea with nobody to implement it (converges to 0).
Y = value of the investor's cash investment
Z = opportunity cost value of your labor (i.e. if your time is worth $50/hour and you will have to put in 1,000 hours to earn your share at future time t, Z = $50,000)
Your fair equity is ( Z - salary ) / ( W + X + Y + Z ).
Many nontech people get pretty upset when you come up with this formula and turn the crank- they think their time is worth more, their idea is worth more, and your time is worth less. (Or they didn't think, they guessed or willed it to be different).
Please try the formula approach and let us know how coming at it with a formula (where you fill in the numbers together) did for your negotiations.
Based on what you described, it sounds like you're coming in at founder level, so you should be a co-founder and have equity on par with the other founders.