I am just trying to remember the 50 series guages... Its been a while...
I think they are the same as the 60's. There should be 3 bar graphs, the left is sieve loss, the right is rotor loss and the middle is combined losses. There should be a line halfway up the middle bar graph and that is where you should run it to 'capacity' (providing it is calibrated properly) One bar coming up is very little, or should be. It would probably be easier to read thru the manual on how to calibrate them rather than me try and help of the top of my head, I haven't ran a 50 since 03. (not sure if its the same as mine to calibrate)
With the tailings gauge, I generally like to run it showing bars filled in about a quarter of the way up the guage, but everyone has different opinions. Obviously some crops and conditions will cause more tailings material than others.