Fire Factors in Bag the Hun

Somewhere there's a guide to calculating Fire Factors for aircraft in Bag the Hun but I can't find it, so I've had to work it out from the aircraft data tables in the rule book and using some basic number crunching. The system I've come up with seems to fit the existing Fire Factors from the tables in most instances, although I haven't compared all of the aircraft against my numbers to see if it's a universal fix. 

The Fire Factor base numbers look like this:

FF 1 = Rifle Calibre Machine Guns e.g. .303 Brownings

FF 2 = Heavy Calibre Machine Guns e.g. .50 cal 

FF 2.5 = 20mm Cannon e.g. 20mm ShVAK 

FF 3 = 30mm Cannon e.g. MK 108 

FF 5 = 40mm Cannon e.g. Vickers S gun 

I haven't gone further than this as it covers most fighter and bomber armament, up to and including some specialist ground attack aircraft. I think this is a workable system and makes it easier to devise balanced firepower capability for those aircraft, like the re-armed Soviet Hurricane IIb's, that aren't in the main data tables. The ammunition limits are, however, something I have yet to work out as it is a bit less straightforward to say the least.