I think this really depends if you want to come up with it's expected value, or you care about e.g. the p50 or p25 or worse scenarios. Worst case is probably too negative most of the time, but occasionally you might choose to join a company that then implodes or has some really nasty problem surface that tanks the stock value. It really just depends on how risk-averse you want to be as to what metric personally makes the most sense.
It all depends on when you need the money. If you are expecting to have to spend the money right after the stock vests, it may be worth 50% of cash due to the chance of a bad market. If you are expecting to save the money for 10 years, it's probably worth as much as cash.