I am looking in my Sniper Log Book / cheat sheet on this one.Depending on the situation, use this situation to get your correct shooting distance for Angle shooting. This is hard to describe without a picture as far as WHY. Think of it this way:

you are on a slope shooting downward you are ranging from your location the the target. that is a longer range then you are actually shooting. Draw a triangle on a piece of paper. straight on the bottom, straight up on a side and the line to the bottom. At the top, is you, and the bottom of the slope is your target. That sloped line is longer than the bottom line. The bottom is the actual distance that the bullet is traveling. That is why you tend to shot higher on downhill shots. The higher the angle, the longer the shot, the more deviation you are goiing to have. I was always taught that it the elevation is less than 2 stories (20-25 feet) then you don't have to worry, if it is more, then you need to do the math here.

SO here is the Formula. YOU WILL NEED TO KNOW WHAT ANGLE DOWN YOU ARE SHOOTING ON. (there is a great device out there that mounts on your scope to give you shot angle)

(G-T) = gun to target distance as per your rangefinder
COS = angle cosign

Angle / COS
10.98
20 .94
30 .85
40.75
50 .65
60 .50
70 .35
80.20

Formula:

(G-T) x cosign of angle = actual distance to dial in on scope.

I suggest taking an hour one day, and doing a cheat card for your rifles that has in 25 meter increments, the correct angle shooting info for your rifle. I kept mine, and my scope adjustment sheet for given distance taped to the stock of my rifle for wuick reference, and so i never was without it.

Hope this offers some help, and does not confuse you too much. Let me know if you have any questions about this.