to me, when people mention ride height,shocks come to mind, every shock needs a certain ride height for the range,[extended,compressed],dimension to work properly.also coil over springs come into play here. i measure ride height from the shock mounting bolts,[center to center].every ride height is different,hope this helps.
There is no exact number for every car. They are all different. Every application will want a different ride height to work well, depending on limitations. Measure at the rear of the front tire from rocker panel to ground, and the front of the rear tire, rocker to ground. With no driver, try to be within an 1 1/2" from the front and no higher than that in the rear. With a driver, I look for an 1". That is just making sure the car is about level. Ride height is staying within that same measurement from front to back, but raising all 4 corners or lowering all 4 corners. Tubbed, big tire cars with ladder bars or 4 links can use a lower center of gravity and low ride height as they will want to lift the front end anyway when adjusted right. Small tire cars, limited on size and material, will want a higher ride height with a higher center of gravity to get the car up on the tire when it leaves. Ballast comes into play as well, and have gone faster with weight added then without when it is added in the right places if a car is lacking wheel transfer. It is application specific, but this is the jist Hope this helps.