Sorry not to have answered this one before but had difficulty getting into this thread.
They determine the refractive error by doing the vision test. Otherwise you could wear a (suboptimal) correction and scrape through.
For instance your true error is -6 you correct with lenses so that you can just manage the 6/6 (20/20) line. For that you need say a -2 correction and you would be fine.
Nope does not work like that.
One goes by the correction needed to get best vision.
FD