True, but as a heuristic I it is safe to say that if you'd use integer units to represent a quantity then you should use "fewer" rather than "less". That tends

True, but as a heuristic I it is safe to say that if you'd use integer units to represent a quantity then you should use "fewer" rather than "less". That tends to capture the majority of cases for using "fewer". Admittedly, the discrete but non-integer-valued cases are a bit more nuanced. (And yes, I do realize that the actual definition is in terms of mass vs. countable nouns.)