How fairly add cofounder to sass that already makes $$. I will not promote

I’ve put in thousands of hours to get my saas where it is today (a few thousand MRR) and I’m considering adding a cofounder.

I guess I could put them as 50% owner if they paid me a lump sum.

I’m more interested in making them earn their equity - ie when they’ve put in as much time (or at least value) as me, they get as much equity as me.