What is slope dope? Angle Cosine Indicator? huh?
The Loadout Room has a really great tutorial on high angle shooting
“When zeroing your rifle at a flat plain, the bullet must create an arc; while shooting at high angle, the arc is slightly different. The effect of this error increases with distance and steepness of an angle to a maximum of 60 degrees. This error applies to both uphill and downhill shooting, meaning that the bullet will always hit above the target, thus you must hold or dial lower than the actual distance to the target.
The greater the angle, the less effect gravity has on the bullet/Shorter gravity distance.
pythagoras-theorem
The mathematics behind figuring out exactly how low we need to hold/dial on the scope are determined by using the Pythagorean Theorem. In mathematics, the Pythagorean Theorem is an equation that is expressed as a² + b² = c² and is relating to the lengths of the sides a,b, and c. For our purposes, the “a” and “b” will represent actual heights/lengths, and “c” is what we need to figure out, also known as the “slope dope.”
More here, its really easier than it sounds