Bullet drop is not linear.
Depending on the muzzle velocity, and ballistic coefficient of the bullet, they will shed velocity- and drop- very differently.
High BC bullets will fly flatter for a greater distance because they shed their velocity slower.
Other rounds, like the popular 7.62 x 39, suddenly drop like a rock after a few hundred yards.
IOW, a bullet which has a flight time of 2.1 seconds, will drop far less than a bullet with a 3 second flight time...and because it will fly in a shallower "arc", will require less elevation adjustment above your zero.
You are correct about the 90 minute adjustment being 45 up/ 45 down.
A 20 MOA base will effectively increase the "ups" to 65 moa.
It's not total bullet drop that's calculated when adjusting elevation. Bullets fly in an arc, and it's calculated based on the minutes of angle above "your" zero distance, that the bullet must be in order to obtain the correct arc to intersect the target at the intended range.
As an example, one of my rifles requires 13 minutes of "up" elevation at 600 yards, with the rifle is zeroed at 200 yards. I've obviously eaten up some of the elevation adjustment for the 200 yard zero to begin with.
Last edited by tobnpr; July 6, 2012 at 08:16 AM.