I wouldn't class him really as a strong player, he can hold off the like of Whittaker, such as in his goal against Rangers, but I'd say that was down to him reading and reacting to the ball first and catching whittaker unaware.
I cant think of a time right now where I've seen him hold his own against defenders using pure strength. Usually it's a case of backing into them, knocking the ball on then using his speed to beat them.
For now I'd say an 80 is fair or 81 at a push. Now he's a celtic player it'll be easier to judge his play week in week out once he's given a run off games.