Johnston's accuracy stats aren't terrible on face value. I agree it was the manner which he missed them that sticks in the mind. They didn't include out on the full stats either, he would have had a fair few of those.
I remember an interview with him where he said over the course of his career he lost all confidence in his technique, and if he shanked one early he was generally in for a shocker. Wasn't bad in open play though, probably because he didn't have to think about his action too much.
I remember going to the old Junction Oval with my old man to see the Cats play the Lions. Johnson had an absolute 'mare. Couldn't hit the side of a barn. There was a moment when a free kick was given to a Cats player in the forward line late in the game (we were flogged - Wilson killed us, again..) and the player was unable to take his kick (injured). My Dad shouted out "Give it to Johnson!!" which was brought a few laughs. The Cats faithful used to tear their hair out when he played.