Another tip for near zero gas loss when changing the drain on a less than empty tank: put a short rubber tune over the vent line sticking out the bottom of the fuselage and pinch off the other end with a spring clamp. This is the ONLY time you want to stop fuel flowing from the tank due to a blocked vent!
This is probably completely obvious to most VAF folks, but I don't remember reading it in the 13 odd years I took for my ultra slow build. You can imagine what happens when you take the drain out, take a little more time than you thought it would take to put one thumb over the hole...and discover the replacement drain is juuuuuuuuuust out of reach of your other hand.
I heard it from a friend.