How do I measure "Ride Height" for a '72 Barracuda? And what is it for a stock set up?

Factory ride ht is the difference between
from ground to bottom of lower control arm where it pivots, specificly use the adjuster arms bottom at that point.
from ground to the bottom of the lower ball joint.

Pictures and drawings in the shop manual and probably also in the MTSC at the Imperial club.org

Lots of info there.

These are for a '73 Barracuda if it helps at all? I am not sure what if anything changed from the '72 models;

http://www.hamtramck-historical.com/ama/_1973AutomobileManufacturersAssociation-01.shtml?load_img=29

http://www.hamtramck-historical.com/ama/_1973AutomobileManufacturersAssociation-01.shtml?load_img=30

Make sure that the gas tank is full and all the tires are inflated properly, maybe add the weight in passenger you plan on hauling I add between 1/2 to one more inch in hieght for it to settle some later