These stats have been done for the entire year. Tony Trabert's 1955 season is the best so far. In the Open Era, Borg in 1978 is the best with over 66% for the entire season. Federer's best is 61% for a few years.
McEnroe in 1984 was at 65.32% which is the third highest of the Open Era next to two years by Borg. I believe Borg averaged over 65% for a few years which is like hitting .450 in baseball or shooting a 55 in the Masters Golf Tournament for one round.