It's based more on intensity, personal preference, training history, and what sort of routine you're using.
Intensity, as in when you use a belt is relative to your own abilities, not some numbers set in stone. Personal preference is also an issue, a great example is comparing two of the best deadlifters in the world, Konstanin Konstaninov who pulls near max weights beltless, and Andy Bolton, who wears a belt starting at 40% (although for him 40% is ~400lbs)...so there isn't one right answer.
Training history, or more specifically, if you have had any back issues or injuries in the past, being more liberal with belt use would be a good idea. And it can also depend on what sort routine you're using...for example, right now I'm following Sheiko, which has a tonne of volume with squats and deads, so I'll use my belt more often than I would if it was a lower volume routine, like 5/3/1 for example.
Some people will won't use a belt for anything below 85% or 90%. Personally I'll use if on lower intensities (like 80%, or a higher rep set of 75%, it varies depending on how I feel) because (in addition to the reasons stated above) I feel that the reps are higher quality (Ie. better technique, speed). I generally put my belt on the set before I want it, to get the groove down.
So you'll have to figure out what works best for you. Try and get in at least a few warm-up sets beltless...but also remember, in the words of Jim Wendler "there's no prize for most weight lifted without a belt, but there's plenty of pain killers, so when in doubt, belt up"...or something along those lines.