What you need is
Buffalobob's Pythagorean gravitational Vector Analyzer
I am having Ealy Summer Discount sale at the current time so I can get you a good price.
seriosly, this stupid thing takes 30 minutes to make and cost maybe 15 cents. It doesn't break, it doesn't get misaligned. It is accurate enough for shots to about 1500yds. You will need a small cheat sheet of angles and drop factors to be put with your drop tables.
I know the picture is not very clear, being as it was taken with a disposable camera. But if you want to make one just email me and I will talk you through any minor points.