So I just realized today that I have been mistaken in how I've been thinking the refueling in the pitstop works. I thought the number you set the fuel to in the fuel setting for the pitstop was the amount it would add to the tank. It's not, that is the number it will go too when you pit.
The way I thought, for example, if I knew I was going to have 20 litres still in the tank at my last pitstop, and I knew I needed 40l to get to the end, I would have had 20 set in my fuel setting, thinking it would add 20 to the tank and give me the 40 I wanted to get to the end. Wrong, if I was above 20 when I stopped, it would add nothing, or if I was at say, 18, it would just take the tank to 20. So to add 20 to the tank I actually have to have the fuel set to 40.
I just tested this so I hope it makes sense!