Excuse me if I mis-understood anything, but I thought you had to pull the front of the vehicle between the A cones? If your vehicle has a front mirror (?) can’t you use that to see where they are in relation to the bumper?
I’d be a “purist” as far as Pete is concerned 
but that seems the best way to me?
I wouldn’t expect the cones were able to be moved? 3 feet is not insignificant. I thought their positioning was strictly controlled?