It is personal and if you never took kava before, it could be that it takes 2-3 times until you notice an effect. Many people do first time. I had full effect first time.
I would suggest to use a scale and do 30-35gr for first time. Regarding the amount of water you have to take into account, that some of the water stays in the kava root. So you end up with less than 8oz. 30gr on 8oz of water will be a very potent kava drink.
Reuse your kava 1-2x !