I have a steel rule graduated in 1/64th of an inch. It is practically the smallest measurement I can make and I try to work to 1/64th tolerances.
Six foot 4 inches is: 72 inches plus 4 inches: which is 76 inches. 76 inches times 64ths is 4,864. You then devide the number of 64th by the scale to give you a size in "scale" of the item. You can convert this to metric to get a 'figure" size. Here are rough equivalents of a 6'4' figure in a couple of popular scales:
Scale 64ths Inches Metric (mm)
1/20.3 239.41 3+ 47/64 96
1/24 202.67 3+ 11/64 81
1/32 152 2+24/64 61
1/48 101.33 1+ 12/64 40
1/56 86.86 1+ 23/64 36
1/64 76 1+ 12/64 30
1/72 67.55 1+ 4/64 27
Then we go to this picture:
We know Lincoln is six foot four. If we print out the picture and measure Lincoln, we measure 6 inches. We can then calculate what scale the photo is (1/12.67) and working back and forth we can fiugure the height of the other two chaps (Pinkerton on the left and Genl McClernand on the right). This would make them approximately 5 foot 8". (Red is the known measurements, blue is the measured size on the photo--the rest are calculated.)
Person Height Photo Height Scale Model Height
Lincoln 6‘4“ 6" 1/12.67 31
Other Guys 5‘8“ 5+ 3/8" 1/12.67 28
You're still stuck with the top of the head/ to the eyes measurement problem, but what is significant is that Lincoln is roughly a head taller than anyone around him. So if you figure size is 28mm (whatever that is) your Lincoln figure should be significantly taller.
The same methodology applies to building models from phots. Measure the size of a known item and base your "scale" or at least relative size on known objects.
I was once in a very crowded theatre hallway with my son who is several inches taller and several pound heavier than I . As we looked across a sea of heads, he turned to me and said, "Dad did you ever notice we are always the tallest people in the room..."