Ok found this ...
Hi Jan, It's just there to decrease/increase the intensity of light. This helps you to put the exposure time into a range which suits you best.
I have the same enlarger and I like to keep exposure times between 10-20 seconds. To ensure this I usually keep the dial at 30. If you prefer longer exposure times dial in a higher number, or visa versa.
A change from 0 to 30 should mean a one stop increase. Similarly, increasing from 30 to 60 means another stop. In my experience though, this is not exactly correct.
I suggest you make a few tests with the dial at various settings, find out which exposure time is most convenient to you, and leave the dial at that setting (unless you have a negative which is dramatically different in density to warrant a change in the dial).
Cheers, omar