That is exactly what I did. I placed a ruler of 40cm on a closet at eye level. I did the thumb trick and found the exact distance I had to stand so my thumb ju

That is exactly what I did. I placed a ruler of 40cm on a closet at eye level. I did the thumb trick and found the exact distance I had to stand so my thumb jumped from left to right of the 40cm ruler. Then I measure the distance I was standing from the ruler. distance / 40cm = your body ratio. So there is no reason to measure length of arm or distance between eyes. We are only interested in their ratio here. You basically run the "thumb trick" in reverse. From the distance and object size => you get the ratio.