An easy way to think about it, as well, is that gravity only works perpendicular (vertical) to the earths level surface. the bullet will only be affected in this direction by gravity, so horizantal distance only affects the time the bullet, or arrow, is in the air i.e. the time that gravity has to act on the object.
The vertical distance to angle distance will always be d=R*cos(theta), where R is the actual line of sight distance, and theta is the angle. easy things to remember are that cos(30)=sqrt(3)/2~~.87 and cos(45)~~.71
so if you are shooting 100yds at an angle of 30degrees, then this is like shooting 87yds horizontal. 100yds at an angle of 45degrees is like 71yds horizontal.
Just remember those two numbers and you should be pretty good with a fairly flat shooting rifle.