For a lot of my realistic drawings, I will use a mirror to get the proper proportions which works great every time, that is if you have lined up the mirror properly.
A chart to better understand the proper angles
I will use 2 grids like this one to get my mirror lined up perfectly. Then I substituted the grids with a photo of Abe and my blank drawing paper.
Be sure to reverse your photo and place it upside down so it will be correct for your drawing. I actually forgot to reverse it for this drawing.
This mirror trick or something similar was probably used by several famous artists that were part of the St. Luke Guild.
