Horizonal error;
Understanding rifle cant includes understanding physics, but the most errors come from not holding the rifle bore axis and scope axis on a vertical plane
www.ssusa.org
Level the reticle;
ACCURATE SCOPE MOUNTING MADE EASY A level reticle is critical to long range Accuracy. For longer shots a canted (or unlevel) reticle will pull or push the bullets trajectory off of vertical center. It gets worse the further the distance increases. A level scope means that the reticle...
www.realavid.com
I have found that placing a bubble level on top of the elevation dial does not assure a level or plumb reticle.
Using the info in the first attachment:
The midrange trajectory of the pointy 140 grain Hornady ELDM bullet, having a sight height of 1.7 inches, MV of 2950 fps, at 1000 yards is close to 90 inches (my 6.5-06). The estimator for a line of sight thru bore is 4 times the mid-range trajectory or 90 * 4 = 360 inches (dimension PT). The calculation for a 3 degree cant would be the line of sight displacement * sine of the cant or 360 * .052 = 19 inches horizontal. The vertical displacement is negligible. This is another factor to consider when shooting deers & such at long range like 1000.
For a shot at a pop can size rodent at 650 yards with a easily overlooked 1 degree cant using a 87 Vmax at 3475 fps at 650 yards - the mid range trajectory is 26 inches > 26 * 4 = 104, sine 1 degree = .017, 104 * .017 = 1.8 inches. Another factor in addition to wind. The rodent target would be somewhat less than 4 inches wide - wait until they are belly to dirt before taking the shot - most misses are left/right.
Upon leveling the reticle install the bubble level thingy on the scope. Placing a level on top of the elevation dial/knob won't work so good if the reticle is tilted in relation to the adjustment knob/dial. Some of my Leupold scopes are affected with tilted reticles.