Cosine rules for uphill and downhill shooting aren't accurate.

The cosine rule says that at 600 yards at 60° is 300 yards at 0°. Which it is, if you are measuring straight lines. But we aren't.
Type it into a ballistic calculator and see how far off you are. Eyes left, -23" 60° at 600 yards, -16" 0° at 600 yards. Off by 7".
Because:
Gravity is no longer operating at 90° to the bullet, so gravity does less (a fishing rod at 60° sags less than a horizontal one, OK?).
Your sights are higher than the bore, the angle changes the zeroing geometry.
The bullet takes longer to get there, and slows down.


So, you have to use a computer. A protractor won't do.