That's a good way to explain this. Logically, it doesn't make sense that something less than one would be plural, but if our only two choices are "equal 1" or "not equal 1" then it makes the choice clear. Thanks.
Is it also worth pointing out that 'a four kilogram turkey' (no S) weighs twice a much as a turkey that weighs 'two kilograms' (S)? (I did anyway ;-)) You can generalize this into a rule about adjectives and nouns - any student care to try?