I'm not sure where they came up with that number, as it's a little on the dry side. 60-80% is normally what's recommended. Keep it too dry and airborne dust becomes a problem, keep it too moist and you start having problems with condensation in the winter, moist litter blowing off ammonia, and markedly greater bacteria growth in the coop. Last winter I kept our layer barn at 80% through manipulation of the exhaust fan run times. That conserved the most heat without keeping things too wet. If the humidity rose to 85% the conditions were miserable in there and the windows were so wet that it looked like somebody had sprayed them with a hose. 80% kept things cozy without excessive moisture.
I have another humidity gauge out on our breezeway so I can compare the humidity inside the coop to the humidity outside the coop. What I'm wanting to see is that the coop humidity isn't higher than the outside, then I know I've got enough ventilation.
ETA: I found this link suggesting 40-70 percent as a good humidity range:
That's not a good comparison. It works in the summer when you are ventilating heavily or have the coop wide open to keep the coop roughly the same temperature and humidity as the outside conditions, but in the winter as you start closing things up to conserve heat it becomes two separate environments. Relative humidity varies with air temperature. If you have two different air temperatures, the relative humidity comparison goes out the window.
As the temperature gets colder, you can cut back on ventilation to conserve heat. As temperatures drop you can keep cutting back on ventilation to maintain the temperature in the coop at a given level until the ventilation is insufficient to remove the moisture from the coop. At that point the relative humidity in the coop becomes the controlling factor and you need to maintain that level of ventilation at the expense of the heat.
The only way to increase the heat at that point is to add supplemental heat. Adding supplemental heat has several benefits. Obviously it will warm the birds, decreasing feed consumption, but it also "dries out" the air. As relative humidity is a measure of how saturated a volume of air is, and warmer air can hold more moisture, when you warm the air the relative humidity drops. By warming the air and lowering the relative humidity you can cut back on ventilation even further, while still maintaining temperature and humidity control, assuming there aren't other controlling factors like ammonia levels.
Wow...so much variance in responses!! I can only say that I was happy if the humidity in my coop stayed below 60% last winter, and was happier when it stayed around 50% or lower, which it generally did. I'm used to dry and dusty in there (almost no rain since June in this area), so that aspect doesn't bother me...