Pyro, you can still have a far more accurate gunnery model even without real numbers per gun and mounting.
My proposal would be to start measuring the power of the guns, it should be easy to find that information, weight of the round, muzzle velocity and ROF would be the factors.
You can start with the less potent gun, probably MG-FF with the known 1 mil dispersion, then make the dispersion of any other weapon be a function of its power relative to the 1 mil dispersion gun (MG-FF?). It would be ideal to know the dispersion of the less potent gun and the dispersion of the most potent gun, and then scale any other dispersion depending on the each gun power between these two values.
Once you have the "base" dispersion of every gun, multiply by 2 or 3 (Hohun?) that dispersion for guns in half wing mountings, and multiply by 4 or 5 for manned guns.
About air combat and gunnery, if you learn to aim, yo will easily predict where the engine of the enemy plane will be when the round reach the target with the target moving, not stationary. Some gunsights were well marked for that purpose. As far as you can follow the target with the sight and make the target "stationary" relative to a known mark in the sight, you will hit with a 0 mil dispersion weapon and with the very first round (unless the target changes violently its course or the turn is so tight that the target is obscured by the nose of your plane). It is like the old "trick" of aiming torps with PT boat against a moving ship, just make the moving ship "stationary" with your PT boat course and launch the 0 dispersion torpedoes, and, if inside range, you will hit always.