The free night certificate (FNC)is 35K points and you can supplement it up to 15K points to book free nights 50K or less.
I’ve used the free night certificates in Hawaii, Anaheim, NYC, Seattle, London, Whistler, and Kananaskis.
Points are generally valued between 0.7 and 0.8 US cents each - so any room where the cash price is over somewhere between $245 and $280USD for 35K points would be considered a decent return. Alternatively, you can factor in the annual fee into those calculations if you choose.
One caveat, you cannot use the FNC as part of a 5th night free with points. (Ie. The 4 nights must be paid with points only- a FNC won’t trigger the 5th night free). But you can use it as the 6th night on the same reservation. That’s usually my go to move. I get three FNC per year (Amex MB Personal, Amex MB Business + Titanium choice gift for staying 75 nights/year)