Back in the day, there was a website called Score Hero. It was to try to find the 4- and 5-star cutoffs in the Guitar Hero games. The way it worked was you took a picture of the song completion screen, which logged your score and stars, and submitted it. There was a floor and a ceiling, and the goal was to raise the floor and lower the ceiling until there was a 1-point difference, which would give you the exact cutoff. This was incredibly difficult to do, as you had to figure out exactly where and how to mess in up very specific ways during the song. But to get the most accurate result, you wanted the lowest ceiling possible and the highest floor possible. Where the floor and ceiling meet is where the actual result lies. Kind of like a much lamer version of the sandwich theorem. The goal was to get the highest floor possible.
21 is the highest floor so far. It is almost certainly not the absolute floor, but we don't know what that is, so that is the highest floor so far.
Also all this reminiscing got me to wonder if the site is still up. It's gone through radical changes, which makes sense since the original mission is long since obsolete, but it still has me recorded as setting the absolute floor on the 5-star cutoff for Crazy On You. I was frighteningly good at Guitar Hero.