The normal cerebral blood flow is approximately 50ml/100g/min or 700 ml/min,
which is roughly 14 % of the cardiac output. 

Cerebral Blood Flow = Cerebral Perfusion Pressure / Cerebrovascular Resistance

Cerebral perfusion pressure (CPP) is defined as the difference between mean arterial pressure (MAP) and intracranial (ICP) or central venous pressures (CVP), whichever is the highest

Imagine it's the BP overcoming the pressure inside your skull/brain to get in to the brain tissue.

MAP = [ (2 x diastolic) + systolic ] divided by 3. 

The reason that the diastolic value is multiplied by 2, is that 

A CPP less than 70 mmHg can lead to a rapid decrease in jugular venous bulb saturation (normal range 65%-75%) because of increased oxygen extraction. 

In TBI = increase in mortality and poor outcome when CPP <70 mmHg for a sustained period.

Pressure autoregulation maintains CBF at a constant level in normal brain in the face of the usual fluctuations in blood pressure.

Normally autoregulation maintains a constant blood flow between MAP 50 mmHg and 150 mmHg. 

Blood Supply, Gross Regions, Blood Flow, Perfusion Pressures & Autoregulation

METABOLIC AUTOREGULATION

Regional blood flow is linked to brain metabolism. 

Carbon dioxide can have a significant influence on CBF. 

As pCO2 rises, vasodilatation increases CBF (when reduced vasoconstriction occurs) 
When PaCO2 <3.3kPa there is no further reduction in CBF. 

Sympathetic NS not too important (CVR 5-10%)
Autonomic NS = v.important in MAP control & CPP, but small contribution to CVR (5 – 10%).

Cerebral Metabolic Rate for O2 (CMRO2) 

The volume of O2 metabolised by the brain and is normally around 3.5 ml/100g/min 

Sources: 

propofology.com

Dr. David Lyness

The CPP should be maintained at a minimum of 60 mm Hg in the absence of cerebral ischemia, and at a minimum of 70 mm Hg in the presence of cerebral ischemia

Here, you can see how CBF increases/decreases with various mechanics in the skull.