I have a question about shot distance....today I had a really nice gobbler come in behind me and present an uphill shot that was roughly 20 yards. I was up a tree about 20 ft. and the gobbler was actually above me....the hill is quite steep. Needless to say, I shot and missed...short. I recall reading somewhere that there was some kind of equation for figuring out the way that inclines affect a shot....is this so?

Any insight is appreciated!