I think you'll find the average sub playing time is rather less on average than a quarter of the game (even more so with Tilson).
Also defenders are more likely to be brought on if a team is winning and forwards if it is losing.
All in all, it really is not an exact science and I know, without statistics, which of our players can play and which can't.
All very interesting nevertheless.
My quarter was a very rough estimate and you're almost certainly right on it being less, even under Sturrock who is somewhat more proactive with his subs than his predecessor.
Agree it's not an exact science, but it's still a remarkably strong correlation to what I would consider our strongest side. The only one that surprises me is Evans over Morris.