The Spitfire gunsight is pretty straight forward, you need to adjust only 2 rings, the base (wing span) and the range ring. To set those rings properly you need to know only 2 things, your opponent wing span (Base ring) and the range of your guns harmonization or convergence ie where in space the bullet will converge.
Once both rings are set you only have to shot once the aircraft wings fit inside the sight symbol ( the 2 horizontal line each side of the gunsight.)
Knowing your convergence is primordial to use your gun to it's maximum effect.
On the subject of gun harmonization they're is multitude of books, bio or historical research that tell story where pilots had there armourer or engineer had there convergence adjust and change from the manufacture. Bio of Malan or Beurling come in mind.
Here a documentary on George Beurling @ 28:21 it explain how Beurling had is convergence change by his mechanic. I know it's only a History Channel documentary but there is plenty of reference about the subject
https://www.youtube.com/watch?v=bGnxGqEEbOc