Just to clear up a couple of misconceptions...
- Re: using pressure and altitude in a ballistics app. If you have uncorrected station pressure then you always keep altitude at zero. If the pressure has been corrected (to what it would be at sea level - typically what you get in a forecast) then you have to enter the altitude so the app can undo the correction. In other words, it needs to know what the real pressure was at that altitude.
- You can't expect good results if you multiply the range by the cosine. That's what's known as the Rifleman's Rule. Use the Improved Rifleman's Rule, which multiplies the drop by the cosine, e.g. if you have a drop of 17.1 MOA @ 600 yards and are on a 40 deg. slope, multiply 17.1 * .77 (COS 40) = 13 MOA. It won't be as good as a ballistics calc, but will be much closer.