The thing to remember is that your Points for Playing formula, set on the Game tab, is specific (unique) to that tournament. You can change it for any particular tournament to compute points differently. The "Tournament Scoring" formula, set on the Stats tab (in the Filter), is a "common" formula that computes a score for each player, for each tournament, using the same formula for all tournaments. It makes it easy to change the formula in one place and have it affect all of the tournaments for which you are computing stats.
If you are using Points for Playing, just modify the formula for the 10th tournament (for example, just add "* 1.5"). If you are using the Tournament Scoring formula, well, it can't distinguish one tournament from another so there's not really any way to give the 10th tournament more weight.
To count only the top 10, you'll need to use the Tournament Scoring formula and then set the Overall Formula to something like:
sum(top(7, scores))