Cofounder implies is that there is a new company founded and that the person involved is someone you trust enough to build a business with. Anything else is just a "what goes on the business card" distraction from real work.

If you're negotiating terms, then it's a finite pie model and you're in the hiring business. That's a distraction from the core business mission - it's a means to an end -- [but an often necessary one]. Founding a company is about grabbing a slice of an infinite pie, it's outward focused. That means bouncing ideas off each other.

I'd be wary of anyone who is looking for the title. That's resume building, not business building. I'd be wary of anyone who is negotiating for a founder role. Founding is about doing not getting.

On the other hand, I'd be wary of anyone who wanted to be a founder without equal equity. Anything else creates friction on the relationship between founders. 50/50 equity is simply a way of testing whether it's a cofounder relationship or not.

I'm not saying you should give away half your company. But I'm not saying it isn't the right business decision either. It's a hard problem because the decision is about whether or not to take a leap of faith.

As a final thought, 10% or 50% if the person you are considering is not the right person, the business is going to suffer. Be equally selective in either case. Pick people you want to be in business with.

Something closer to 50/50 is what Paul Graham always advise for later co-founders. Forget the past, imagine how both of you will work equally hard from now on. But remember that he is talking about startups that aim high. Maybe a billion dollar company, at least worth tens of millions. If this is also your ambition, to get your company from bootstrapped regular income to a $100 million dollar company, then go for a 60/40 deal (I am not convinced 50/50 is really desirable). But remember to have a good lawyer and take all precautions about vesting and if this person it really the right one.

But if you aim at a good, healthy, profitable small company; I would just hire someone. Even if it is another more junior person.