This is the quick and not–so–dirty method I use. I've found it quite handy when the tactical situation gets chaotic.
Plug the formula into your
fx-5000F or smart phone and Bob’s your uncle. Heck, grab a pack of Lucky’s, a cup a joe, whip out the old slip-stick and do it old school!
Don't let the trigonometry intimidate you. After a couple of patrols you may find yourself solving this in your head.
I hope you find it useful.