It's a lot more art than science. When we were drying my hull with the hotvac system, we did some scientific tests. There are three different meters that work on different principles. All of them were designed to measure moisture content in wood, not fiberglass. Bits of metal in the FG will drive them crazy, as will any metal fixture on the inside of the hull. I did things like take 50 measurements in the exact same spot one after another. We compared the three different meters in the exact same spots. All read differently, and different readings all over the place. I even took a piece of my hull that we had cut out for a bow thruster, dried it in the oven for 24 hours, then tested that (you can tell I'm single).
All I can tell you is that the best it's going to do is give you relative measurements from one area to the next. It won't pinpoint anything except metal.