This is where my other rule comes in: "I am either a freelancer or a cofounder".

To be your cofounder you must also convince me that this project is something I want to spend the next several years of my life on and it must be great enough to convince me to drop other projects and start working on this full time. I don't believe in part-time cofounders, that way lies burnout and failed projects.

And despite all that, equity still isn't tasty so I will need to earn at least some of my normal rate anyway.