Anywhere in the 70's or 80's Fahrenheit.
And I don't understand what does it mean to "not use all the digits". They are used, not just for the narrow range he cares about which is the comfort zone in his particular region of the world.
In Belgium it is between 15˚C and 25˚c all the year with few extremes on either sides. Nobody need 3 digits to tell if it's warm or not.
So Belgium is a perfect example of a place where Fahrenheit would be better.
(And, for the record, "low 20's" °C would be 68–73°F, which is a perfectly useful range, just with a slightly shifted boundary than what both of us are used to.)
where |s| := the cardinality of s
Only slightly less cardinality, a phrase that takes longer to say, and much less precision.