I've read that you want to have no less than 30% free. That's the rule of thumb I go by.
If I may, semantics: none of these Macs have "hard drives." The generic term is "storage." I know what you mean and I'm not poking fun at you because I still use that term a bunch. But I try to say "storage" to be correct as much as I can. That's 13 years of being a copy editor for you.
It's really no big deal. I just have to unleash that old copy editing energy somewhere.