I was told the dosage was 800mg(400mg 2x) a day until Ovulation then cut back to 400mg a day...
Unfortunately taking that much didn't work out well for me - every time I have tried to take it - my cycles shortened to 19-20 days - which I don't need!!!
I have read lots and lots of success stories about Vitex. All you can do is try!
I bought mine on Amazon