Back to the topic, the most effective simple calculation I have used that works well (without using PDA's) is:
1.) Determine range to the target
2.) Determine your bullet drop in inches/cm
3.) Measure the angle you will be shooting
4.) Multiply your bullet drop by the cosine of the angle
5.) Apply the correct adjustment for the new bullet drop information
This method is best used on a rifle that has a 100m zero. If you have zero'd out further, it is less accurate. Bryan Litz has a very accurate formula that accounts for angle shooting, but it is much more involved than the one above I gave. The one above will work in most situations very well, and is simple.
The horizontal distance is the only thing to take into account as gravity is the same shooting uphill or down
Im not being an ass here, just trying to pass on something I learned, because I used to think the same thing. Gravity does effect the bullet differently depending on whether or not it is going uphill or downhill. If a bullet is shot horizontally on a level surface, gravity's only effect on the bullet is to pull it directly downwards away from the line of sight. Now if a bullet is fired on a downhill angle, it is being pulled less from the line of sight and gravity is also helping it to maintain its speed. When fired uphill, gravity is still not pulling the bullet as far off the line of sight as horizontal shooting, and is also acting to slow the bullet down. Due to gravity not directly pulling the bullet off the line of sight, the bullet drops less over distance and this is why rounds shot at angles, are always high.
