I've got a Eureka Auto Drybox AD-101, bought 9 years ago. Problem is I can't find the instruction manual anywhere now. Does anyone know how to calibrate this model of drybox?

There is a dial, with off, L, M and H on it. the 'off' is located at the 3 o'clock position. The L, at the 9 o'clock position. The M, at the 11 o'clock position and the H at the 1 o'clock position. Also, when the dial goes above the L position, a red light will come on, then go off some time later. It is written RH control at the corner of the dial panal.

The humidity in the drybox is maintaining around 55%, is this too high? What would be the optimal humidity level?