As I recall, with U.S. fighters such as P51, pilots had a choice of 300 or 250 yard harmonization. I've been using 300 on my .50 cal rides, but am going to try 250 to see if that is better, since I am shooting in closer in AHII. (I'll leave my P47s at 300 - better for strafing). Years ago someone in AH calculated the the best compromise for .50s, taking dispersion into account, was 240 yards. Don't know if this is still relevant.