I don't exactly know how the distance system works, but here's the way I see it. If you are closing on a target... which is usually the case, an average pilot would start shooting AFTER they see their magic number. Let's say the magic number is 400 out... most would shoot after they close within 400, not just before it hits 400, because the marker still says 600. So basically when your bullets hit, the target isn't 400 out, the target is actually closer than 400 if you are closing. This is why I set mine for around 300 on each plane, both MGs and cannons.