Fleet special orders "When fuel less than 20%" doesn't appear to kick in until fuel is at 10% or less.
I can't reproduce this one...
..just to be sure, I'll set up some situations where fuel is between 10 and 20% tonight when I play.
I set up a controlled condition, where I detached a parasite with 15% fuel and told it to join the mothership if fuel less than 20%. In the turn afterward it joined the mothership's TG.
However, it seems when out in the field sailing from place to place, that it gets to where it wants to go before detecting the fuel situation and ordering a return home.