With the analog dry cabinet, you turn a dial to control the amount of dehumidifying.
There is a battery-operated digital hygrometer (measuring humidity in %) fixed on the door, but this is a form of visual feedback to you only. The dial is the one that controls the humidity. So you will need to set it to a mid-point (approx) and let the humidity stabilize over maybe 24 hours. Then you check again. If too low (I think about 45-50% is pretty all right) or too high, just adjust a bit and then leave it again to stabilize and so on and so forth. Once you get the optimum RH, it's very unlikely that you'll need to do any more adjustments for a long while.
Or if you feel even more lazy, you can get a digital dry cabinet, in which you preset the RH you want (in %) and the electronics are supposed to take care of the rest, much like you set the temperature digitally on your aircon remote control.