There should be a mark or a line stamped into the body of the reservoir approximately 3/8 from the top. Fill to that line and you'll be full. Allow the level to drop too low and you will start sucking air into the system. If that happens the steering will make all sorts of noise and the fluid will appear pink and frothy.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.