Do you turn ON your aircon 24 hrs? If yes, you don't need a dry cabi. If no, do note that when temp goes down, RH% will go up and vice versa. So, when you turn on your aircon (temp drop), the RH% will go up, the cabi will take a while to bring it down again. When you turn off you aircon, the RH% will drop and take sometime before the RH% will go up again. If I remember correctly, 1degC change will cause 2~3%RH change.
This may be a reason why you think that the cabi can't be stable.
Anyway, just maintain around 45%RH, it depends on the size of the cabi and power as well as the stuffs you put inside. For me, to bring the RH% down, I need to turn to 2 o'clock and when it reach around 45%RH, I will turn it to 10 o'clock. You have to try to find out what setting is best for you. You setting should be for the longer period between your aircon ON or OFF
Good luck.