I understand that largish Silicon Valley generally offer their employees options like these. What's the conventional wisdom on the uhm-valuation to use for such

I understand that largish Silicon Valley generally offer their employees options like these. What's the conventional wisdom on the uhm-valuation to use for such employee options as a fraction of what they might get from a Series A/C/E/G round or an IPO? Note: I'm not suggesting that Elon Musk uses that conventional wisdom, just asking what it is. For all I care, he might think that he should put the value much higher than the employees consider fair so they'll stretch themselves, or much lower so they'll be loyal, or both. I'm just asking what that conventional wisdom is.