It can be tricky. I've seen HIE/BML start a night at like 40/40, clear out, and stay calm. Then they procede to fall to something like 25/25 before finally succumbing to freezing fog. Lower els can tap into colder dews that drain down into the valleys as hill/mtn surfaces radiate. If I see MWN running very dry in the evening despite a near saturated start at those sites I always figure they'll radiate like mad anyway. It seems to be difficult to "fog out" once you get into the mid 20s too unless you're near a body of water or have laying water handing around. I mean, how often do we see fog with temps below 20F? Only the most extreme rad pits seem to do that. Diamond dust is pretty rare south of NNE too.
But yeah, minus continued advection overnight, the afternoon dewpoint is a good estimate. If you're confident you're going to see radiational cooling MOS can be tough to beat too.