I guess i don't understand your concern. It's only for comparison purpose. It's not like he's building it around that measurement. I'm not trying to argue it at all, i just know that on a custom build like this having something to compare it all to helps to see where you end up.Different air pressure and different brand tires can have a fairly large margin of error. 1/2" to me seems like a fairly decent amount when taking on a build and wanting done right. Maybe not exact Mm, but why leave a 1/2" to chance?