225 myself...pick where you normally will fire..with your fighting style...and set it to that. I set myself up for a lot snapshots in close...therefore I set it at 225. The 50 cal planes I fly, I set at 375 or so...I shoot farther out...not nearly as many up close and personal snapshots.
Once you figure out the timing of it all...and the drop of the shell...you can adjust for other distances based on where you have your convergence at....just takes some practice.