I'm going to go out on a limb and say that when your fuel meter says 1/1 it means your tank is full. Like others have said 1/2 and R will mean different things on different models. Just drive it until you reach R then fill it up you'll know how much it takes. I'd wager it'll be constant on a single car. Same with 1/2.
1995 E420 - retired
1986 420SEL - retired