Why worry about it? If it's showing F5 two days out, your upgrade will probably end up clearing. If they do manage to sell all five F seats in the last two days before the flight, well then good for AA.

Although I haven't heard the 20% "rule" before, it does track fairly closely with my experiences on most flights. There will always be exceptions... and I'm guessing it's not just based on route/day/time but also based on other factors that we can only guess at. (Maybe this is the week of a big Star Trek convention in LA, and since AA knows why we fly, they know that there are a lot of hard-core Trekkies in the Dallas area.)

It's four hours (not two), and while this is usually the case, there are exceptions here too. vasantn, in particular, has documented multiple instances where AA released all available F inventory into X well before the four-hour mark.