I realize that the practice of throwing seafood after making mistakes is very popular locally, but I do not encourage it. However, if you must do so, it is much more productive to aim at the offending player, rather than at the offending note. If the second hornist ducks as you throw, then it is quite natural that you should hit above your target, or even miss entirely. (And even if you miss, it is likely that the third or fourth horns will receive the benefit of your exertions.)
If you throw clams at your music, and always hit above the target note, then the chances are you are failing to allowing for transposition, as well as the effects of range and air resistance. This problem can be solved by methodical practice. Get out your Kopprasch, choose an etude, select one of the recommended transpositions, aim those clams very carefully, and then fire away! You should notice a big improvement right away.
An effective way to prevent clams from sliding down the page is to turn your music upside down.
Gotta go,
Cabbage