1) Don’t look at the revenue but the profit. So take the 40k and deduct your personal time (e.g., 100 USD/hr) and the costs. Let’s say that leaves you at 20k. Mulitply with 25 for a conservative 500k valuation for your side-project. 2) What happens when they let you go? Do you get back the 100% stake? What happens if they merge it into another product and “yours isn’t making any more revenue”?
I might suggest the following: 500k for your side-gig vesting in salary/stock over 2-3 years (and guaranteed unless you do something evil). Plus your market rate salary.
As soon as you join them, revenue share doesn’t make any more sense because you don’t have control over the product any more. And should you be penalized because they screw it up?