According to soccerbase, 72 wins out of 159 so 45.3%.
ETA: In the league along its 48.51% according to Wiki which would give him the second highest percentage ever amongst permanent managers. David Hay is first with 66.6% however this was in division 1 so basically, when talking about league win percentage in the top division, Stuart McCall is the most successful 'Well manager of all time...