It's zeroed by holding it at arms length and having a 6' friend stand 100 yr away and slide the finder to where he fills the opening then knot the string so when you hold the knot in your teeth the distance is always the same arms length.
I was thinking set up a 4' target (ave ht at shoulder of mule deer according to wiki) set a second knot, a third for 2', etc. That way a quess-imate of height (@ shoulder) could give a pretty quick distance (within a yard or two)...especially for someone like me that like being traditional (and cheap..I can't afford $$$$ laser rangefinders)
Hell, I was even thinking cheap golf rangefinders.