i like using the T17 that T/C makes for these ML'ers... I just run 1 pre saturated patch up and down and then it takes roughly 3-4 dry patches to get her clean. After that I run there seasoning patch down which is just bascially bore butter. Thats after shooting the BH209 also
kinda odd for 1 shot to be that far off. Are you certain you didnt jerk the trigger? How far was the target?
either way, i wouldnt worry to much about it since you got the group you were looking for

Some days ill shoot great and then others I just have a bad day which ticks me off, but we have to let those worrysome days go LOL
any of those shots has a dead deer down