At those ranges the angle isn't going to be much of a factor at all unless it is over 30degrees.
There are lots of online ballistics calculators that you can play with that will give you a good idea of how much difference it actually makes.
A good angle cosine indicator
gives you just what you need to adjust on the fly in the field.
Here's a pretty good, very simple article on the subject.
Angle Cosine Indicator Aids Hunters « Daily Bulletin
Many of the more modern and even very reasonably priced range finders will give you the "true ballistic range" with the push of a button.