To calibrate measure the diameter of the circle you cut out. The recommend you use calipers but I only have a ruler and the Mk.I Eyeball. I measured the circle at 22.5 MM
So I find 22.5MM on the ruler on the cutter arm
and apply the calibrating sticker with the tip pointing to 22.5 on the scale.
*NOTE* If you have a sharper eye than I did at the time, you will see I have the sticker pointing to 23.5mm on the scale! Oops! I will soon find out how this will affect my cuts!