I think they have to be measured on the car because of all the variables he and everyone else has mentioned. Tire pressures, driver weight, chassis differences, engine angle and on and on. A 40 degree header measured in a jig might measure 39 on one car and 41 on another depending how the motor is tilted or whatever the case is. I agree they need a solid tool and a flat place to measure each car (while theyre on the scale comes to mind) but it really can't be done off the car in my opinion.