The effect of gravity is what we are overcoming with the flight of a projectile.
The effect of gravity on a projectile is perpendicular to overall curvature of the earth, not perpendicular to the slope of the shot.
For perspective, if you are on a cliff 400 feet above your target, and your target is 300 feet from the base of the cliff, your line of sight, and the distance your projectile travels would be 500 feet....but the effect of gravity on the projectile would only be 300 feet. Without understanding that you would assume your sighting would require compensation for 500 feet, and you would hit high.
To answer your question, the easiest is to just use a range finder with angle compensation software on board. I just bought one from Doug, very reasonably priced.
