I thought I'd share a little on dry cabinets as there does not seem to be conclusive answers on the site. I just got a larger cabinet over the week and this is my conclusion and observations on the whole frustrating affair of setting it up.
Ideal storage humidity
40%. After searching and observing everywhere high and low, I accept this is the 'ideal/standard' humidity for photo equipment.
Moral of storage humidity: As long as it is not above 60% as specified in most camera manuals.
Size of cabinets affect the humidity at any given dial setting. I.e: more space = more air = higher humidity = higher reading. For example, for smaller cabinets, if you set the dial at 12 o'clock, the humidity reads, say, 45%. A larger cabinet at the same setting may yield a higher reading.
Tip: Check the power consumption of the cabinet and you can identify how efficient the dryer is. I.e, don't count on setting the dial at the same position to get the same humidity level for 30L and 60L.
Adjusting humidity level
A new setting will take roughly a couple of hours to stabilize. But I observed that you would only get a true reading overnight. This is the source of all my frustrations actually. So, adjust the humidity without any stuff inside and wait overnight. Check and adjust accordingly. The humidity also changes according to the temperature: lower temp > lower humidity.
Useful tip! (For a certain popular brand of dry cabinets) Wondering what the A, B and C mean? A is 40 - 50% RH, B is 30 - 40% RH and C is 20 - 30%. BUT these are all aprroximates.
Items that asorbs more moisture like paper, cloth, camera pocuh, etc, will throw the cabinet humidity off the scale to as high as 60-70% This is normal. (The cabinet humidity with door closed and placed in my living room, the unit off, read a whopping 95% one day)
Patience is the game here. I took almost a week just to get the humidity I wanted and the reading stabilized.
What to put in
There's no rule here - Wet stuff get dried, dry stuff gets drier. Batteries are fine since high humidity corrodes the terminals, so it's ok. I throw all my photo stash inside there.
Hope this helps to eliminate frustrations in setting up a dry cabinet!