What should the % of moisture in wood be for proper turning ?
Not an easy question to answer. No single answer. Too many variables.
Is your wood green or dry?
Is the wood blanks covered in wax?
How long has the wood been in your shop?
Where do you live? Relative humidity varies a great deal between locations.
What is the season? Relative humidity also varies a great deal between the seasons.
Are you turning spindles or bowls? A big bowl blank takes a long time for moisture change.
If the wood is green you need to twice turn, once to rough shape, put aside and allow to stop loosing moisture. Weighing is the best method to determine when the wood stops loosing moisture. You can then turn to final shape.
If the wood is dry and has been in your shop for several months then you may be able to turn. Depends on the size of the wood. When in doubt, leave the wood longer before turning.