It's not a matter of knowing the angles, it's visualizing and 'feeling' the position of the
guns, taking account the response lag. It just has to be like you play a ball game, you
don't memorize any numbers, just get a feel for it.
When you get good at it, you can maintain fire on aircraft even when they're taking
evasive action, and at times like these, knowing angles doesn't do much good.

once you get used to the guns, you'll know ALL the angles.

I constantly use the US 3"/70 and 5"/38 guns, and whatever height the planes fly, i can usually hit
them on the first salvo.

http://trainworld.us/NF2/English/Guns.html
That pretty much covers everything (The golden angles is the angle at which you will
fire at aircraft flying at normal level the fartest away you can)

It is not a guessing game, though to a dilettante like yourself it might seem like one.

I've reached the point where over half the time (not including planes at DEFAULT height),
I hit the plane on my first shot. It takes a bit of practice, a bit of talent, and a bit
of trial and error with estimation.

Get the feel of the gun, angle and the plane.

Its not a guessing game, it is an experience game......

SO HOW to do it?? (except for Bugged plane) you need to know the high of the
plane comparing with It's shadow. And distance from your ship.

different gunz comes with different angle, but there are a few angle that you need
to memorize. If you have master those angle, AAing will be easy.

And you need to know how fast the plane are going, how further you need to aim.

Experience are an important fact in the game......

It's below your Golden Angle, and above 10 degrees. Beyond that, it's individual. I can
guess an angle after 2 shots, max. First is a test fire to see if I'm aiming too high or
too low, and second is the kill of the plane.

From my experience, it is mostly a guessing game- however, going with the KM 5"
guns(32 golden angle) I've noticed that you typically can subtract 4 degrees from
the angle per noticeable drop in altitude if the planes are circling around the normal
golden angle's arc.

Each gun will be different, but as you use em you'll get better.