One approach you can use: If you're working 1.8x fair rate, work out what 0.8x your salary is & imagine you were an investor with this sum to put to work - how

One approach you can use: If you're working 1.8x fair rate, work out what 0.8x your salary is & imagine you were an investor with this sum to put to work - how much equity would you require? That's a starting base for your negotiation. Good luck.