Turning clockwise towards the 'Red zone' lowers the RH; likewise, turning towards the 'Green zone' raises the RH.
The RH (& temp) do fluctuate slightly (adjusting to the environment) even if you do not open the cabinet often.
From my experience, RH of between 45-55 is ideal for photographic lenses; especially so if your lenses are those with in-built AF motor.