I have one of those moisture meters as well, and in my experience, they don't work well. Ideal moisture content is 6-9%, but how does one arrive at that number? Short answer for me is experience, I guess. If I've cut wood myself, I let it season for a minimum of 3 years at an RH of about 60%.
I also judge a staves readiness by it's weight. If it's considerably lighter than it was green, and the weght seems to have stabilised, then I start to work it. This is all for staves, mind you, as kiln dried boards are ready to work the day you buy them.