For all the times that I painted portraits, I found 2 ways to measure proportions: (1) use a stick, ruler, brush or similar device, or (2) simply start painting.
Measuring and calculating the proportions of a portrait is important—it will make or break a painting.
The problem for measuring occurs when models move—this is very natural—the measurements that were perfect seconds or days ago can be totally out of place.
So, for me, measuring too much takes away the experience of painting from life, so I don’t measure much. Once I am happy with the head size and the overall placement of the figure, it’s time for my army of brushes and paints to start the battle.
This is not to say that I don’t measure. I am constantly measuring while I am painting, but much of it based on judging whether or not I am able to capture the likeness.