I live in the wet PNW, but my my home is the same as many people, around 68-75 40-50%RH
When it rains a lot a very seasoned stave can gain a gram or 2. A gram of water is 1 cm3, so when a stave is losing a few grams a day, that's a lot of water to me.
This topic comes up from time to time, everyone has their own ways of doing it. I like a scale because it gives me the best idea of much water is in the wood.