Text in force at CS29-12: from CS29-0, effective 14 Nov 2003.
If the unusable fuel supply for any tank exceeds 3.8 litres (0.8 Imperial gallon/1 US g allon), or 5% of the tank capacity, whichever is greater, a red arc must be marked on i ts i ndicator ext ending from th e calib rated zero read ing to the lowest reading obtainable in level flight.
ED Decision 2003/016/RM · Source