OK, that makes a little more sense. You originally talked about a "points" formula. This formula would go in the "Points for Playing" entry on the Game tab, and would calculate each player's Points value for a tournament.
The Tournament Score formula, on the Stats tab, works pretty much the same way as the Points for Playing formula on the Game tab, so you can use the same formula. This will compute a value for each player, for each tournament that is included in statistics calculations on the Stats tab. The Overall Score formula is used to combine each Tournament Score for a final, overall value.
For your points formula, this should work:
(n - r + 1) * if(nr > 0, 7.5, 10)
If you test this with a tournament of 10 players, the player ranked 1st place will receive 100 points. They'll receive 75 points if they have any rebuys.
So, are you trying to compute stats at this point, or are you testing this for a single tournament?