What does %MAC mean?
%MAC (Percent Mean Aerodynamic Chord) is a mathematical calculation that shows where the center of gravity is over the wing.  The calculations is rather straight forward and the generic formula is the same for all airplanes.  All one has to do is to figure out the numbers and plug them into the formula.  

How is %MAC calculated?

Pilots, dispatchers or load masters will calculate the weight and balance with respect to the aircraft datum.  Unfortunately, we need to change where the reference point is from the (datum) to the front of the wing called leading edge mean aerodynamic chord (LEMAC).  There is also a trailing edge mean aerodynamic chord (TEMAC) that is calculated.  TEMAC - LEMAC = Wing width.

LEMAC and TEMAC are easy to calculate for straight wing aircraft.  It is simply the measurement from where the wing starts and ends from the datum.  For swept-wing aircraft it isn't that simple.  As a result, aerodynamic engineers figure out where the average of the LEMAC by averaging the wing root and wing tip.  The same process happens for calculating TEMAC.

%MAC is simply a calculation of how far the CG is from the LEMAC.  If the CG is at the LEMAC, we can say it is at 0% MAC.  CG at TEMAC means the CG is at 100% MAC.

In the picture below, we can eyeball the %MAC to be around 10% MAC.



Here is the generic formula:

$$ \%\mathrm{MAC} = \frac{\mathrm{CG} - \mathrm{LEMAC}}{\mathrm{WingWidth}} \times 100 $$



The formula is great because all the numbers are relatively the same for every airplane.  For example a 13% MAC means that we are most likely at or close to our forward CG limit.  This should instantly give the pilot a sense of how the airplane is going to respond versus a pilot trying to comprehend  what a CG of 2,341.0 inches aft of datum means for that particular aircraft.  The last number can be different for every aircraft type but 13% MAC will be the same.  The only difference is where the forward CG limit is located for that particular aircraft.



A little background, and perhaps an engineer can expand on the thought.  The basic weight and balance formula is $ \mathrm{Weight} (W) \times \mathrm{Arm} (A) = \mathrm{Moment} (M) $.  What may not be obvious is that we can use this formula in figuring out why %MAC is important.  



Take a look at the picture above.  There is a moment between CG and CP and another moment between CP and tail down force.  These moments must equalize for the airplane to be in equilibrium.  If the CG moves forward, Moment #1 gets larger and to compensate the tail down force must increase.  We can increase the tail down force with nose up stabilizer trim.  The same analysis can be made when the CG moves aft.

If this is the case, there is a specific pitch trim setting for each %MAC and airspeed combination.  Most jet pilots will set a specific takeoff pitch trim setting for takeoff based on %MAC so that if the pilot loses an engine after $V_1$ the trim will be set for a speed around $V_2$.