What percentage of blood?

You have to define your terms. One could argue that in one course of circulation 200% of the blood goes to the heart because of the dual circuit, one circuit providing 100% to the lungs (plus a little that goes to nourish the bronchi). At rest, the typical cardiac output is about 5L/min (the total blood volume is about 5L) and the brain typically receives about one sixth of the total output.