Really? Because if fewer indicates a countable quantity, then you are counting thirds. And the only non-negative integer less than one is zero.
I've never heard this rule. Do you have a source? I'd be curious.
Which might be exactly what we want in dialogue
Well, I really wasn't arguing less versus fewer, but simply pointing out that grammar check needs you to be accurate before it can be accurate.
But thirds of what? If you take one third of all married couples away from one third of all married couples, you have zero. But if you just take one third away from one third, you still have two thirds of one third remaining.
One third must be one third of an actual number. To find one third, you first have to know how many married couples there are, and then how many get divorced, One then can you say one third get divorced. You can also then give anyone who asks the specific number that one third represents, or that fewer than one third represents.
And "a" third isn't a writing rule, or a grammar rule, it's math. One third is an actual, existing thing in math. A third isn't. Because of this, "a" third isn't programmed into grammar check.
It might well be what we want in dialogue. Using "a" is common, and I suspect most people do so in speech. But even dialogue comes down to how readers think about the character. When quoting some statistic, or anything like it, it might be best to use the proper terms, if you want the reader to believe the character knows what he's talking about.
