Oh wow, I'm surprised the conversion hasn't been shared. One level tablespoon is six to seven grams of medium grind kava root, dried, most of the time. A scale would be preferable because different grinds weigh more or less, and humidity factors dramatically - a kilo of fresh green kava may dry out to be two hundred or three hundred grams.
My general rule is 7g for a rounded tablespoon, four tablespoons for an ounce(28g).