Calculating the vertical center of gravity requires raising one end of the vehicle and measuring the weight increase on the wheels at the other end. 24 inches is a good height to use. Unless you raise one end of the vehicle at least 12 inches, the calculations may not be as accurate as you might like.
The weight increase is defined as the weight on one end of the car (left plus right side) with the other end raised a known distance minus the weight of the same end of the car when the car is level. It is always a positive number.