Sweet. Then I say that your equation will work wonderfully. See if I got this right:

As people opt-in to the tournament, or after the tournament begins, we track the average ITM of that tournament's field:

(Total number of all-time finished ITM for all the members who are currently opted in)
/
(Total number of all-time opt-ins for all the members who are currently opted in)


This gives us most accurate Total Average ITM number.

Then when the tournament is over, we use your equation with the refined Total Average ITM number:

(Number of Players)X(Total Average ITM)X(Place Finished*)= Strength of Tournament Points
*place finished would be inverted: EX 30 players means 1st is worth 30, 2nd worth 29, so on

This would give us a total score for each individual user for each tournament modified by the strength of that tournament.

Then simply create a LeaderBoard that shows 1st place having the highest score and so on. In order to qualify for the leaderboard, you have to have played in a certain number of tournaments (say 25?)

Make sense?