I think this assertion is the source of confusion. I think the multiplier ends up being worth way more than 28.5%.
The exact number depends heavily on the order that the kingdoms are included in the computation.
At worst (lowest values first), I have it at 48.9% (326 glory). At best (highest values first), I have it at 113% (467 glory). I also got 219 glory as the baseline.
You can check my math and let me know if I messed anything up: