The meter is not something you can just put in and use and rely upon, unfortunately. I have one and got some useful information from mine, but only after I was able to calibrate it against some other independent measurement of soil moisture at depth (like the eyeball test when you're repotting and have the plant out of its pot, or poking a finger/chopstick in there a couple of inches). You can't really be sure the soil is dry when the meter says so, at least not without a couple of extra steps first.
The way the meter works is it measures conductivity, which will vary depending on the purity of the water, the composition of the soil, etc. So a reading of 5 will pretty reliably be wetter than a reading of 3 for example, at least in the same pot. In that sense it's very useful. But comparing numbers in wildly different soil mixes is probably a bad idea. I keep all my plants in more or less the same mix (which is 50% pumice, 50% organic) so I can pretty easily compare one pot to another without being concerned about conductivity artefacts.
Anyway, what the meter calls "dry" may not actually be dry if you put in a finger or judge by weight or whatever (use any other reliable way to judge soil moisture). My advice would be to use the meter when you are pretty sure the soil is going dry but not necessarily bone dry (ie. a good time to water) and use that moment to calibrate the reading to a known fact in observable reality. Whatever it says then can be used as a threshold down the road for when it's time to water. Does that make sense?
