Quak, these guys aren't making scientifically base recommendations that account for barrel harmonics. If you are fit and muscular my calculations suggest 21.674". If you're getting on in years and a little soft try 20.5985"

Yer welcome.

Pecos Kid

I am..........disturbed.

Concerning the difference between man and the jackass: some observers hold that there isn't any. But this wrongs the jackass. -Twain