We drain a whole litre of fuel before each sortie for a fuel sample in case of something happening. Before the next flight I chuck the litre of back into the 200L drum we are refueling from. You just proved after a 3 hour sortie that there was nothing wrong with the fuel you just used. So why throw it away?
A litre seems excessive. I'm not saying its wrong, just that it seems unlikely to be more useful than,say, half a litre. Have you ever found anything in the second half litre that wasn't in the first?
David