My co-founder wants to split the company 65/35. What's the correct proportion to split ownership in a company?

Owners of corporations can own any percentage of stock.  Assuming that the two of you should own the business 50/50, one way of facilitating stock for future investors is to have the company maintain the unused stock for later use, sometimes called treasury stock.  If the company is split 65/35, then your partner will own 65% of the company.  Now, he could transfer any amount of his stock to a future investor, but that would be up to him.  Also, I am assuming there are no restrictive stock agreements and the future offerings will not be substantial enough that you need to register the offering. There are several other ways to set the transaction up that are somewhat more complicated, but the easiest way would be for the company to own the unused stock and then it can be sold to investors at a later date.