Good question.
A missed tournament is simply a score that the player will not have. For example, if there are 7 tournaments, and a player participated in 5 of them, he will simply have 5 scores.
Dealing with this depends on what your goal is, and there are several ways to do things.
The trick is to use the "n" variable (or "numberOfTournaments") in the Overall Scoring formula. "count(scores)" in the Overall Scoring formula tells you how many scores this player has, whereas the "n" variable in the Overall Scoring formula tells you how many tournaments have been included in the calculations (how many tournament files passed your filter).
So, if there are 7 tournaments, this will average each player's highest 6 tournaments, regardless of how many scores the player has:
average(top(n - 1, scores))
If the player has participated in only 5 tournaments, it will average those 5 tournament scores. In other words, it will only drop the lowest score if the player has participated in all tournaments. Otherwise, it doesn't drop any.
This, on the other hand, will average the player's scores minus the lowest score, regardless of how many scores the player has:
average(top(count(scores)-1, scores))
In other words, it always drops the lowest score.
Now the ugly part. The "scores" variable holds all of the players scores for the particular set of tournaments. If the player does not participate in a tournament, they don't get a 0, they simply don't get a score.
As an easy example, if there are 3 tournaments a player might have the scores [50, 75, 100]. A player who only played in 2 tournaments may have [50, 100], NOT [50, 100, 0].
Why does this matter? The average of [50, 100] is 75, whereas the average of [50, 100, 0] is 50.
Therefore, the problem occurs if you want a missed tournament to count as 0, and thus affect their overall score. Currently, there's no way to do this (that I can think of). What probably needs to be done (off the top of my head), is to add an additional variable, like "allscores", which is the same as "scores" except it contains 0 values for tournaments in which the player did not participate.