Everything always goes back to the basics... And the Basics for me takes me back to an incident that happened in Texas during the 1970's. It wasn't a hunting experience, but a defensive survival one. I was not there, however the incident is a Testimony regarding the importance of correcting the sloped distance to target, to the corrected for gravity distance.
There was a bad guy on top of a building shooting down at everyone he could with the intent of killing them. Many innocent people were killed and it took a multitude of armed Citizens to finally stop the murdering.
In this particular scenario, picture yourself in the defensive posture. You are armed with a Remington 700 chambered for the .308 Win. You are 300 yards out, aiming and shooting up to the top of a sixteen story building on a 45 degree angle. If you do not correct your sloped distance to target to the corrected for gravity distance, your bullet will be eight inches above your point of aim. Since you can only see approximately sixteen inches of your target, the correction is mandated and absolutely necessary.
When you Zero your rifle you usually do it shooting straight out, flat. The full force of gravity is indeed bearing down on the projectile. In this zeroing situation, you will adjust the sight's height above the bore for this particular constant; "that being the full force of gravity."
However, when you begin to aim up or down on an angle, the force of the gravity depreciates and the "Constant" (full force of gravity) changes, (lessens). But the sight height above the bore that you dialed in while zeroing, has remained the same.
There are several other elements that should be addressed in order to correct for the change of the "Constant" (force of gravity) and the way that I can explain this best to go through the three methods of correction.
The original method is called the Rifleman Method, and it is merely multiplying the cosine number of the angle that you are holding on to the sloped distance to target. You are all probably familiar with this as your objective is to figure out the "Flat" or straight line distance to target. Because you zeroed your rifle on a flat plane, right? So, 300 yards multiplied to the cosine number for 45 degrees (.7) equals 210 yards. 300yds X .7 = 210.
The next and more accurate method is to utilize the "Improved Rifleman" method. That is where you multiply the cosine number to your "hold" as depicted on your data card. For example, I shoot a .300 Win Mag; and my hold for a 600 yard target without aiming on an angle, is 11.25 MOA. If I was aiming on a 30 degree angle, my cosine number would be, ".87." In this case, I would then multiply the cosine number for 30 degrees, which is .87, to my MOA hold of 11.25. 11.25 X .87 = 9.78 MOA and this is what I would dial into my scope, or if I had a reticle that subtended in Minute of Angle, I would hold on the corresponding mark.
The most accurate method is to utilize Ballistic Targeting Software such as the one manufactured and distributed by "Night Force." This is because the software includes in its calculation, the bullet's weight, the Ballistic Coefficient of the particular bullet that you are utilizing, the sight height above the above, the muzzle velocity and the site in distance. With these set of elements, in addition to the software’s trajectory validation sub-routine, it is capable of calculating the bullets own unique deceleration curve, and deliver to you, The Shooter, a very precise hold.
With the Night force Ballistic Targeting Software, it utilizes either the Cosine number of the angle or just the angle. Many Hunters appreciate utilizing the "angle" that they are holding on because they can use Night Force's Software on their desktop computer to create a data card with the angles that you might be holding on as a cross reference chart in the field. As an example, if you are aiming at a deer 500 yards out, and you are holding on a 20 degree angle, you look at the distance to target on the left hand side of your card, then look at the angle that you are holding on along the top of your card and you will be able to locate your hold for the shot. This is very fast and eliminates the need for an additional calculation in the field...
The following link depicts Sniper Tools Angle Cosine Indicator. Once attached to your rifle, that is where it will stay.
aci-htm Sniper Tools