If your co-founder put in significant work before you started, then you should expect less than 50/50 ownership. It sounds like you both put in equally develop

If your co-founder put in significant work before you started, then you should expect less than 50/50 ownership. It sounds like you both put in equally developing the idea, and equal risk of your effort having no payoff (a very high risk for any startup). I can't see any reason from your description that you should expect less than 50/50 split of the reward. (Maybe 40/60 depending on some contributions before you got involved).