I was never very clear on how player ranks were determined. So I did some research and created the Foldit rankings wiki page.
My big discovery is that the main rankings are based on your total "global points" in the last 120 days. I'd previously thought that rankings were based on the last 40 or 50 puzzles. (On the other hand, there were 49 or 50 puzzles completed in the last 120 days....) Feedback 989490 documents the 120-day window, and also the 30- and 7-day "month" and "week" windows.
There were some other discoveries. I found the global points formula is documented in the technical section of the Foldit FAQ. But what's not mentioned is that the number of soloists competing is used for both the soloist and evolver rankings, while the number of groups competing is used for the group ranking. The smaller number of competitors for the group ranking means that the points awarded drop off much more sharply as your rank decreases.
Another finding was that beginner puzzle points do not count toward group ranking.
I've asked Inkycatz to update the FAQ with a mention of the 120-day window used in rankings.
So far, the 120-day rankings I've looked at have checked out OK, in that the total global points reported on fold.it match the results for the individual puzzles. I haven't been able to successfully verify my own "prediction" ranking yet, but the number I get is at least within 5 points of the number on the website.
I also want to look at group rankings. I've noticed that the numbers reported on the groups page don't match the numbers on each individual group's page. For example, the groups page reports 3,754 points for Beta Folders and 3,065 points for Anthropic Dreams, but their individual pages show 3,115 and 2,926 points, respectively.
I've asked Inkycatz to update the FAQ with a mention of the 120-day window used in the main rankings.