Honestly the clipboard guy is usually just checking off boxes on a sheet that already has your car's info from the booking, so what looks like "measuring" is mostly confirming what class your car falls into. I've watched them do it a bunch of times and they glance at the car, glance at the paper, mark a category. Where it gets fuzzy is the edge cases, like a lifted truck or a car with a roof rack that pushes it into a different size bracket, and yeah in those moments it's kind of a judgment call. But I wouldn't call it guessing, it's more like they know the categories cold and just match your car to the closest one. Where I do agree is that the "measurements" are not precise, they're just bracket assignments, and if you argue your car is smaller than they say, nobody is pulling out a tape measure to settle it. So your proof might show the process is sloppy, but not that the guy has no idea what he's doing.