Humidity matters so little as to be negligible. Play around with the aforementioned ballistics progams and you'll see what I mean.
Altitude matters quite a bit, but you might be time/money ahead to get an altimeter/barometer like a Kestrel
4000 (other models work, its just what I happen to have)... what changes is the barometric pressure (gets less with higher altitudes), but it can change in some weird ways due to local conditions, terrain, etc. so it can help to know what it actually *is*, not just what it 'should be' for a given altitude. Of course, then a PDA becomes pretty helpful... and the list goes on ;)
In general, you're going to start seeing a noticeable decrease in drop out past 400yds. My 1k zero for a .308 Win 155gr VLD @ 2960fps changed from 31 moa @ 800' ASL to 24' moa @ 6600' ASL.