The comment about no two people having the same zero is bunk. Take this for what it's worth. My older brother was about as different from me as could be. He was left handed, about 6'1", 250-270#. I'm right handed, 5'9", 150-165#. I sighted his rifles in all the time and he then fired a couple of shots to confirm-into the same group.
I used to do sight-ins for pay when I was younger. $5-10 plus ammo was normal. I've shot a LOT of different rifles and never had anyone find a discernable difference in zero. A hard kicking rifle may show some vertical dispersion if a second shooter doesn't resist the recoil the same as the first. That's why a rifle zeroed with a lead sled won't give the same POI when fired from the shoulder.