What to set it to

The official guidance, quoted

The EPA's mold course states it plainly: indoor relative humidity should be kept below 60 percent, ideally between 30 and 50 percent if possible. That range exists because mold, dust mites and several common pests all require humidity above roughly 60 percent to thrive, and holding a space below that removes the condition they depend on.

Note the range is 30 to 50, not a single number. Anywhere in it is fine for the mold question, which means the choice within the range is about comfort and cost rather than health.

Why 45 to 50 rather than 35

Because the last few percentage points are disproportionately expensive. The drier the air, the less water there is available to condense for each unit of energy spent chilling the coil - so run time climbs steeply as the target falls, and each increment removes less water than the one before it.

And 40 percent or below in living space is dry enough to be noticeable: static, dry throats, and in winter it can dry out timber and joinery. There is no benefit to offset that, because the mold threshold is 60 and you are already well clear.

Do not trust the built-in reading too far

Built-in humidistats are approximate. They sit in the intake airstream rather than in the middle of the room, and they are not calibrated instruments. A five-point disagreement between the machine's display and a separate hygrometer is ordinary.

The practical response is a cheap standalone hygrometer placed where you actually care about the reading, and adjusting the machine's target until that hygrometer says what you want. If the machine reads 50 and the room reads 57, set the machine to 43 and stop worrying about which one is right.

The setting that costs the most

Continuous mode, on a machine that is undersized. Every figure on this site's running-cost pages assumes the unit reaches its target and idles; continuous mode removes the humidistat from the loop entirely and runs the compressor regardless.

A 50-pint unit in continuous mode is $64 a month at the US average rate rather than $32. There are legitimate uses - drying a space after a flood, a crawl space with no useful humidistat position - but it is not a setting for ordinary running. Running cost.

Seasonal adjustment

In winter, a target that was sensible in August can cause condensation on cold windows, because the same relative humidity at a lower temperature means a higher dew point relative to cold surfaces. If windows are streaming in January, drop the target by five points.

In an unheated space in winter the question is usually moot: below about 60F a compressor unit is barely working, and below 41F it has effectively stopped. Switching it off for the cold months is not neglect, it is recognizing what the machine can do. Icing up.