You know, there's about 25 millimeters to the inch, and I've never seen anyone measure anything with more than 16th-inch precision for any construction job - maybe for very high-end cabinetmaking people use 32nds, but generally millimeters will give you more precision than you need, certainly if you're using a carpenter's pencil, which makes marks thicker than the smallest scale on a ruler.

I have no problem dividing things by 3 in metric. Rounding up to the next millimeter is going to result in an error equivalent to a couple of human hairs. If I'm working with wood and I need a perfect fit I'm going to be using sandpaper long before that.

Seriously, do you imagine that construction workers and craftsmen in Europe, Japan and the rest of the world spend their days ina state of helpless anxiety because of their inability to divide things with sufficient precision, or do you think they just get on with it and make buildings and furniture as good as any you can find in the US? The construction industry didn't collapse in the UK, Ireland, or Australia when they adopted the metric system.