I think y'all are trying too hard.
There's too much chaos to create a hard-line value for x. You do what you can do to reduce the wear to a reasonable level, but you don't need to take it to the nth degree. By the time you get to that level the number of variables are too great.
We know that keeping the tires the same circumference, roughly, will enhance longevity. How much is academic. It's easy enough to accomplish with minimal maintenance. If tire rotations cost $50-$75, I'd be weighing the value of having them done every 5,000 miles. See: Point of No Return.
