If you go to this link:
Under "Anti-Aircraft Gun Information" on the right side, click the arrow next to "United States"
After the chart comes up, look for the gun you use - Notice the N, L, D, or A next to the gun name which dictates which gun you are using.
Look down the column "Normal" under "Angle" and correlate it with your gun. That is the "golden angle" - the angle of fire at which you have the best chance of hitting aircraft with AA shells.
For instance, if you use the 5"/ 38 cal Mark 38 D's, the "golden angle" is 40 degrees.
I apologize if I was not supposed to link to that website.