Ah, Grasshopper. When you learned the answer to that question, you will have no more questions.
Seriously, I have yet to receive a good solid answer to that one. Some say same as bottling, some say half. That's it. There is no definite answer. You choose. I personally use the full amount per carbonation calculators. As temperature and style and amount to carb are different, 2/3 cup is too rough for me.