Pat, can it be you are a bit sloppy, occasionally? If you have 36 liters of gasoline and the fuel consumption is 20 liters per hour, you have (example) a save flight time of 1 hour plus 45 minutes reserve plus 1 liter non-usable fuel at level flight - don't forget the margin for bank angles. At latest (!) you are down in the woods after 2 hours, but this is not what FI wants to hear - at least if it would be me. Pilot guesstimation is not about straight forward math, its about knowing the machine you fly - even though your instructors calls it calculation
. Got the difference?
Maybe a short story based on some cut&paste from the thread so far?
(Level Attitude): "OK Pat you say we have 40 Litres of fuel on board. Is that total or usable?
This aircraft uses approximately 25 litres per hour. For today's exercise we will be flying for approximately one hour. As you know we like to land with a least 45 minutes worth of fuel remaining. Can we go, or do we need to go to the Pumps first?"
(You): "And just to answer your mathematical question no we wont have to go to the pumps to fuel up we have a sufficient amount of fuel to fly for an hour."
(Me): "Ok Pat, I understand. Let's roll the plane back to the hangar and we do some ground work. You are not fit for flying today."