I've got a little level thingy which is a budding project, but it tells you the ratio for the loss... what you do is angle it at the target, read off the ratio, and multiply that by the range. So if it says 0.9 its 90% of the range you should dial in.
So for very close targets, say 10 yds, that makes them into 9yds, which we all know will mean a miss. For a 50yd target that makes them a 45yd target, which again is a miss. But for a 30 yd target, it means a 27yd range should be dialled, which on that part of the trajectory will probably mean little or no difference.
So you need to know the angle, the cosine ratio and the range, otherwise you'll be like me in italy and guessing