Have both founder sit down and write a list of things they think the other founder brings to the table. Assign an equity value beside each point (with the remaining equity belonging to you). Spend some time doing this, encourage them to take it seriously.
After both of you have completed this, sit down and run through the entire list. Have an open and honest debate about each point.
Once adequate consensus is reached, add up the total % of equity assigned. Hopefully, this number will exceed 100%, if it does not (ie: both co-founders believe they make an outsized contribution), it's probably a good sign you need to go back and restart this process again. Multiply your personal equity stake by the total % equity to get your share.
It seems like a good idea because it avoids making the argument self-serving and talking about your own contributions. Instead, it's about getting the partnership off on a foot of mutual respect & understanding.
Disclaimer: I've never tried this myself, results may vary.