Rogent,
200 is too close for p51 flying in my opinion.
I have mine at 300 all guns. If you think about it I always assume 50 cals tend to do more damage when I'm flying real fast. I open fire at 400 odd and by the time they connect it could be around the 300 distance.
50 cals are good. Some exceptions being hitting 190,s, 109's, La's, typh/temp. These planes seem to sometimes get away with a good solid burst say they get oil hits only and fly off.
I would reserve 200 and less for 303's. Bearing in mind 50cals lose power the further they travel 600 and above will require a plane to be pretty much straight and level.
However I have had some success finishing off kills from 800 zoomed in. You know the 190 types trying to escape. They have to be flying near level tho.
Any shots 800-1000 your talking some good markmanship.