The serious field archery guys will use a "cut chart"
http://www.getoat.com/page6.htm
I have a rough cut chart memorized in my head for shooting 3D as distances are estimated and I believe inclinometers would be considered a ranging device so anlge is guestimated by the SWAG method (Scientific wild a$$ guess). I use that as a third method to guesstimate my uphill downhill shots.
( you can pick up a cheap inclinometer from anywhere that sells back country ski gear , part of your avalanche kit)