This is probably the easy answer. But you need to understand what happens when computing your Stats.
The "Tournament Scores" formula is a place for you to create a value for a player, for a particular tournament. So you can come up with your own method of creating some kind of scoring value (exactly in the same way the Points for Playing works).
The "Overall Scores" formula(s) are a way for you to combine the Tournament Scores values you created into a final value.
If you just want to average a player's points, there's no need for you to create a Tournament Scores formula. However, since the Overall Scores formula is only for combining the Tournament Scores values, you DO need to create values for Tournament Scores. You do this by simply assigning a player's points to the Tournament Scores value. Thus, your Tournament Scores formula would simply be:
points
Or, if it makes it clearer if it looks like a formula or expression:
points + 0
In the Overall Scores formula, you can access the Tournament Scores values by using the "scores" variable. Your formula should work:
sum(top(10, scores))/10
Or you can use the average function:
average(top(10, scores))