Measuring would be MUCH more accurate. As you pointed out not a very sophisticated part.
BUT.... LIABILITY upon failure, Brakes are the single most important part of a car (steering second) if it fails and someone dies (or worse doesn't but is incapacitated the rest of their life) and the failure can be traced back to the piston... I don't want to be on the receiving end of that court case. (Yes I was brought up in California, and it shows)