When dealing with millimeter adjustments it is best to use a caliber. My trusted friend is a proportional divider made by Cesieco. It was a bit expensive, but worth every penny.
Once when I took a sewing class, my teacher told the class it's best to measure twice before cutting. Well this applies here as well and you want to cross reference every measurement once vertically and once horizontally.
1- Walk up to model and with proportional divider's large end measure your models head. Try not to poke him/her!
2- Walk back to easel and hold large end against the canvas.
3- Count how many times the initial measurement can repeat on the canvas. Don't forget to leave room for frame overhang.
4- Let's say you can accommodate twice the original measurement. Loosen the screw on the divider and adjust the marker so that it reads 2.
5- Go back and double check!
6- Mark top and bottom portion of head measurement on canvas and start drawing in straight lines.