Portrait artist deal with millimeter adjustments and this is where using a caliber for exact measurements comes in very handy. My trusted friend is a proportional divider made by Cesieco. I purchased mine throughAccurasee Proportional Divider $12.99 Accurasee Art Products
Mine was a bit expensive, but worth every penny in the long run. It allows me to enlarge or reduce drawings in exact scale and look for errors. I found another more reasonably priced proportional divider on Amazon, which would be a great tool to usejust to get used to the idea of measuring while painting.
How to use Proportional Dividers
You set one end to the proportions of your reference photo, then you try to adjust the slider until the other end corresponds to the proportions of the area you are measuring on your painting. Once the caliber is adjusted you can measure every other corresponding feature.
This tool is an absolute must for any portrait artist and comes highly recommended!