Buy a cheap digital scale that does grams or ounces, up to 5-10 pounds and then weigh each piece and mark it, do this every month, and when it stays the same weight for a few months straight, it is ready to turn round. Once round, do the same for a few more months to make sure it is actually stable with the environment you are in. I tend to weigh the rounds after each cut and then a couple months later before the next cut. I live in New England where it goes from 65%-80% humidity in the summer down to 20-25% in the winter, so the seasons come into play.
HTHs,
Dave