Not an easy question to answer. No single answer. Too many variables.What should the % of moisture in wood be for proper turning ?
Assuming you are talking about final turning, it should be close to equilibrium for your area. For thoses who take a weight it is when the item weighs the same for several weeks. I use a moisture meter and locally (Charlotte, NC area) I turn once it reaches 15% or less.
Equilibrium here is 12-14%.
Here is a link to equilbrium by major cities; starts on page seven (by the pdf).
Indoor is typically less than the above outdoor but my shop is basically outdoor. The only way for me to make it less by a couple of percentage is bring it inside for a few weeks prior to the final turning.