Perhaps the best way is for you to do some self testing. Fill your tank unitl it is full, then drive until the fuel meter reaches the 1/2 mark then fill the tank full again. How ever many litres it takes is how much you have used. The R stands for reserve I think, and mean that you are down to the reserve amount. In my 1986 420SEL the reserve is 10 litres, which means I have used almost 90 litres by the time I get to the R.
1995 E420 - retired
1986 420SEL - retired