I think it should be fine to just hold a ruler up to your finger. The only potential problem might be that the highest tip of your finger wouldn’t actually touch the ruler, but if you don’t want to estimate by sight you can hold another flat surface perpendicular to it to see where that touches the ruler. I get consistent measurements this way.
Done! Wish I had had a scanner handy going in, I’m curious about the digit ratio.
I think it should be fine to just hold a ruler up to your finger. The only potential problem might be that the highest tip of your finger wouldn’t actually touch the ruler, but if you don’t want to estimate by sight you can hold another flat surface perpendicular to it to see where that touches the ruler. I get consistent measurements this way.