Dremel 678 circle cutter is a really useful tool, but not easy to set to a precise specific radius. This thing acts as a calibration ruler, allowing for easier setup of cut radius from 10mm to 200mm. Place the tool at the center hole (yes, the tool), and the center nail at the desired track. Ticks are 1mm radius apart and aligned to the radius line, so you can see them through the cutter tool's nail opening. Added drs_2 which allows for 1/4" tools as well as 1/8". The reference hole is replaced by tha reference tube, which should fit the 678's hole through which the cutting tool passes.