Dale, you'll really need to create a datum plane under the car. This is an imaginary reference point to which height measurements are done. If you look in a factory service manual at the body measurements you will see a datum plane on the drawing. Most technicians don't know what this is because the modern computers have one stored in their data base, and then the computer adjusts to each vehicle. A datum plane could be imagined as a flat surface under the vehicle that all height measurements are based off. I would say an acceptable tolerance in the 60s and 70s would be about 1/4 + -