I’ve tried explaining this on the wiki as best I could but it could still use some work. It goes by total experience in the skill, but blue stars add additional XP requirements.
Say you have 100K experience in a skill with no blue stars, and are comparing it to a skill with 3 blue stars.
Each blue star requires 11.1% more experience, so it’d be:
100,000×1.11×1.11×1.11 (or just 100,000 x 1.11^3) = 136,763.
Then each blue star also adds 20K experience required, so it’d be:
136,763+3(20,000) = 196,763.
So in this example you’d need almost 200K in the skill with 3 blue stars to come out ahead of the skill with 100K experience and no blue stars. To figure out any other relationship between skills you just change the appropriate numbers around.
(If you’re comparing skills that both have blue stars just remember to subtract out 20K xp per blue star before multiplying by the 11.1%’s though, or things’ll get screwed up.)