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 height is measured at the centerline of the axle of the end of the vehicle being raised, and the vehicle must be raised by being supported at this point.