Your eyeball. Assuming level ground and forget the rifle as there's all kinds of different shapes. Different ring heights too. I'm ordering a custom rear bag. Nothing super fancy, a Protector DR. but with a custom height. Cranking my head back hurts my old neck. If you lay flat on the floor face down and try to lift your head to look straight ahead. Ouch! Now push your chest up (pushup like) and the bend in your neck decreases. My current system is laughable. See pic. So how high is you eyeball above the ground?