Measuring on a curve is hard to do unless you have a calibrated flexible curve. This one will hold any shape with a minimum radius of one inch and has a raised lip for inking. One side is fully divided into millimeters and the other side is graduated into inches. The first six inches are subdivided into 32nds; the remainder into 16ths.

Metric graduations are marked in red, inches in black for easy identification on white scale backgrounds.