I'll usually water a bit after the surface soil dries out. Otherwise I'm just going by feel. Once I left it a bit too long and didn't water for maybe 4-5 days during a warm spell. All the leaves wilted but perked back up within and hour or so after a good, heavy watering.
My humidity meter is also pretty cheap so I don't doubt that there are inaccuracies, at least it gives a bit of an indication. Now that it is starting to get colder I've been watering much less, once or twice a week as the humidity is supposedly above 90% most of the time according to my meter.
Hope you start getting some good growth soon!