I think when it says your 400 out it's actually somewhere between 500 and 301 and when it first says your at 200 out from target it's actually 300
I take a lot of shots when it says 400 but not normally in any of the spits that carry 303's. I like to get a little closer and as soon as it says 200 I start firing. I'll try to keep my target in the 200 range using throttle management.
The 16 guns are a little different. Having the 50's maks it possible to take longer more heavy hitting shots. The 50's are on the inside and shoot long and straight so I set them out to 325-375 giving me a good convergence pattern at 450 and below. The cannon being farther out on the wing but not so far that i need to set them out longer I put at 275-300.
I've tried both set ups and like 350 for the 50's and 300 for cannon best. (not shown in pictures) Both cannon and 50's cross at about 250 and again at 325 spreading out across the targets wings from there. The 50's I can still use effectively at 450-500 forcing them to turn.
Hard hitting Spit 8 and 9's