Um... I think that's exactly what I told him. The whole "millimeter" thing in my post was to be able to use a ruler to measure parts rather than use math. If you want something to be, say 25" long in 1/25 scale, then you could divide by 25. But when you start getting into decimals it can get annoying. Or you could simply measure out 25 mm on a ruler, and you got you're 25 scale inches. No need to go all crazy with the math.
The millimeter thing comes from the fact that 1 inch== roughly 25 mm.