Text in force at CS25-28: from CS25-0, effective 17 Oct 2003.
If the unusable fuel supply for any tank exceeds 3.8 l (one gallon), or 5% of the tank capacity, whichever is greater, a red arc must be marked on its indicator extending from the calibrated zero reading to the lowest reading obtainable in level flight.
ED Decision 2003/002/RM · Source