I also have plenty of problems with these gunsights :)
My actual approach is to fix the range to 400yd in the P-51 so that i have an idea of when is the best moment to fire. Only for long range shots i adjust the range, but not in a turning fight.
On the german side, i assumed the usual 400 m is the ideal range, but that's based on the 109's recommended settings.
The world is beautiful, because it's colourful :P I enjoy them all!