The average stack.

[Talking Poker]

As for your original question, Lou, you probably won't like this answer, but here it is:

I see playing the big stack and playing the small stack and the exceptions. Those are the times when I have to (choose to) change my basic strategy in a tourney. So for the other times, when I have an average stack, I just play normally.

If your questions is really, "How do you normally play in an MTT?" that's one thing, but I can't look as having an "average stack" as some sort of exception to standard play. That is what standard tourney strategy is designed for.