The formulas are contained in your Stats Profile. You can create as many Stats Profiles as you want, each with their own selection of tournaments and formulas. The Default Stats Profile creates a score for every player using the Tournament Score formula log((n + 1) / r) and the Overall Score formula (1-exp(-average(scores))) * 100. To use your own formulas, create a new Stats Profile and substitute your own formulas.
On the Stats tab, press the Edit Profiles button. Then press the New button, give the new profile a name, then select the profile and edit the various parts of it (where it looks to find tournament files, which leagues or seasons it accepts, and the formulas it uses).