This is a common thing in field archery as well because you are commonly shooting at up and down hill targets in wooded settings. They are notorious for setting up on extreme up and down hill shots. Doug Williams with Copper John/Stan is probably one of the best field archers in the country right now (behind cuz), and he discusses this a bit over on archerylive.com. Here is his response in quotes:
Cutting yardage isn’t quite as simple as the trigonometry explanation but the formula will get you very close on 90% of the situations. It is proven that shooting up hill the arrow will slowdown faster (slower down range speed) than on flat ground and slowdown slower (faster down range speed) on a downhill shot. On a 80yd uphill shot at a 5 degree incline you will have to add a bit(less than a ¼ yd) to be dead center but on a 5 degree downhill you will have to cut 3/4yd almost a full yard difference between the 2 but on a 10 degree uphill shot you will have to cut ½ yd and on a down hill 10 degree slope you have to cut 2 full yds. The reason for this is the change in trajectory in relationship to the line of sight on an uphill 80yd shot 6 degrees is where the two will balance out between the velocity change and the change in trajectory so you will have to shoot it for a straight 80. After more than 6 degrees the change in trajectory will always be greater than the change in velocity so you will have to cut yardage. On downhill shots the two will never balance out and that is why you will always have to cut more on downhill shots in comparison to an uphill shot at the same slope.
[/align]
You can download ontarget2's free version and can actually print your own cut chart on this stuff. Below is an example of a angle cut chart from ontarget2. It is the graph on the lower right hand side. I have one of these printed for both my 3D and field bow and these charts are dead nuts accurate every time.
Edit: Pic didn't post....here it is.
