The PSI is generated by averaging the opinions of your Top Users.
Criticker stores your top 1000 users, ordered by Taste Compatibility Index, and the first 10 who've seen Nothing are used to generate your PSI.
First, their film tiers are averaged. The resulting Average Tier is then translated into a score, based on your rankings.
At least 3 of your top 1000 users need to have seen the film, before a PSI can be generated.