So let me get this straight...
If I have a zero set on my rifle that is PERFECT at 100 yards and i'm going to be firing at 200 yards, I would increase the elevation knob by 1 click and each click represents an additional 100 yards?
I have a Bushmaster XM15-E2S 20" A3.
No, Each click represents a distance (called MOA - Minute of Angle) that the point of impact will shift from the point of aim.
Which means each click of elevation on the rear drum is roughly 1/2 moa on your detachable handle.
So to go from 100m to 200m you'd need 2 clicks (which can vary depending on the ammo/environmental conditions).
If you had a fixed handle upper [A2, which has 1 moa per click of elevation adjustment) you'd need only 1 click.