Assuming you mean (3 ft / 2) - 12 1/16 inches, it's about 6 inches, which is probably the accuracy you need. But it's exactly 5 15/16 inches, which is also easy to do in your head. Carpenters do it all the time, and even a doctor can.

My head did 36" divided by 2, so 18", minus 12 1/16", so 5 and 15/16". That is harder than working with quarters or fifths of meters, but easier than thirds or sixths of meters.

I don't know which is better. Considering how nothing stops you from doing decimal inches or fractional meters, but people rarely do that, both ways seem fine.