Much like negotiating a salary or company valuation, your objective is not necessarily to assign the perfect value. Your objective is to add a highly motivated partner to your business, feel great about it, and get to work on the important stuff. You want to mitigate the possibility for resentment or the need to revisit this in the future.

There are so many things to consider, how can you find the perfect number, anyway? What is their target salary? What is yours? You've put in two years, but are they super charged up and going to bring new energy into the company that you badly need? What about their professional network? Will they bring future customers, team members or investors to the table? Was the investment you put in chump change for your situation while the new person is investing their kids' college fund?

If you try to solve this problem from the standpoint of determining the perfect value, you could spend weeks crunching the numbers. To Assaf Ben-David's point, this is a psychological problem of how to motivate two people and create a relationship that both people value. Today's company value has little bearing on that unless it's super important to your own motivation and sense of fairness. You could be fine with 60/40 and so could your new partner, and you're off to the races without all the headache.

Just try to find the number and vesting terms you can both live with no matter what happens. If you're both happy and motivated, you've achieved the objective. If you pick the perfect value but neither of you is very happy, you've failed.

There is a lot that goes into picking a co-founder. If you want to chat about this, please reach out.