I've seen different concepts in various articles, where they mention "maintaining 50% field capacity" or around 30% humidity. But how do I get to that information?
There are many resources online that describe field capacity and how to calculate it. The main point is to understand what it is, as then it would make more sense in what you actually measure and how. See below for a few areas you could start:
It all depends on the evaluation period, you should maintain watering and then stop doing it for 4 to 5 days, after this period return to normal watering.