If your exposure times are way too short, like when doing a 3x3 print from a 2x2 negative with a 250 watt bulb, you can set all three dials to say 50 for a neutral density filter, then ONLY adjust the other two, leaving cyan on 50. so if you std filtering is 45Y 50M then your filter dials would read 95Y 100M and 50 C.

Otherwise do not touch that dial.