In my opinion, you should take the difference between their market salary and the salary they're being offered, and consider that an investment by the employee at the upcoming (not past) valuation.

For example if they're in SF and they're hiring a senior first engineer that would maybe make 250k elsewhere, and they're offering them 125k, and they would take the classic 7% for 125k, then 7% is a good starting point. (Of course if they already have the YC investment, then that would go down dramatically)

If that equity vests over 4 years, then frankly maybe 28% is a better starting point.

But what's fair isn't really relevant. What's relevant is what the market demand and supply is. If there's some dolt who would happily take 1.5% as a first engineer ("founding engineer") for a $125k salary cut, then the founders would be idiots not to take that deal. And frankly, if that $125k salary cut gets them their dream job, then maybe they're not even dumb for doing it.