Watching one of the tribute programmes (BBC Icons of football?), it looked like he made it easier when taking penalties by usually sending the keeper the wrong way.
Not sure if there's a statto out there who could give us the percentages and don't really care, but in any case they all went just inside the post, left or right.
The calmness and self-confidence required to be so deadly from the spot, in the highest-pressure situations (League Cup final replay; Scotland v England at Wembley) speaks volumes.
As I recall, we got quite blasé about his ability to beat defenders: 'Skin 'im Robbo!' was shouted with the near-certainty that he would, so we didn't maybe appreciate quite how special his ability was.