Some people use the tire itself for the measurement. I don't trust tires to be that perfectly symmetrical. With the aluminum placed on the tire, it's sitting against variations in the sidewall like big lettering, small lettering, little nibs. You get a hard flat surface with the rotors.
Because the tension in the tape measure might deflect the rubber in the tires using the aluminum arm as a lever. You can measure from the tread in the tire, that wont deflect the tire enough to matter. Measure from the same tread block on both sides. That's what I do and it works fine.
Thanks! notedMore repeatable measurements, they wouldn't be as repeatable if the aluminum tubes were attached to the tires.