This is how I think, how McMahon score is calculated (based on https://senseis.xmp.net/?McMahonPairing):
The initial score depends on your overall rank. Every player above the McMahon bars starts with 0 points. The score within the McMahon bars is reduced for weaker players (lower rating, fewer starting points). Below McMahon bars every player gets the lowest points.
In the just started Tianyuan Nines Title Tournament, the initial score near the McMahon bars looks a little random to me. The McMahon bars are (9k-3k). For example there is a 2k player with initial rating of -4, a 7k with -2 and a 4k with 0 points.
Therefor I’m wondering, how OGS calculates the initial score.
Edit: The screenshots were made before the points where adjusted to fit the special Tianyuan initial points. Therefor the screenshots should show the points as calculated by the OGS system.
The pairing seems to be off as well. It is set to Strength but player of similar strength are scattered over multiple groups.