With wing mounted guns by far the most important thing is shooting and hitting at your convergence setting. Especially if he hasn't seen you , you must make that first shot devistating, that we you avoid a long drawn out dogfight, which attracts unwanted attention.
If the diving 109 is in a tight spiral...aim for the center of the spiral...if its a consistent wide spiral you should be able to tell the next spot he will travel thru. Fire a wall a bullets that he will fly thru it.
Like Zoom said get close, if your convergence is close, but when I'm shooting him I stand off alittle because those Spits blow up real good.
