Well put. Would be entertaining to see the same basic maths with an adjustment for probability of success. E.g. "if you feel the extra confounder increases your odds of success 40% ..."
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.