You’re on the right track.
It comes down to what both parties perceive as “fair”, the company risk level at the time of joining (early days/no cash flow), and the time commitment each founder is putting in (full time v. part time).
As founder of a legal tech startup –we routinely help tech startups deal with these issues through our community of startup attorneys.
Here’s apresentation we recently did on How to Split Equity in a Company:
Hope that helps!