At the end of the day though, I don't think this kind of percentage calculation is useful (entertaining, sure). The human mind doesn't work that way. Similarly I think it's a mistake to bring on a cofounder purely for the fundraising benefits.

The decision to bring on a cofounder should be based on whether they bring critical skills, connections, and if you really want to work with them. IMHO, going with your gut is going to be a much safer bet than putting a number on it and using that to influence your decision.