The magnet is not going to stick, you'll need some type of adapter to thread onto the spindle with a steel base same size as the magnet or bigger, for the magnet to stick.
i haven't checked the caster on mine. Just used the notches/numbers provided. If you want cheap just get an angle finder and lay it across the wheel, long as its not too beat up it will be within a couple tenths. I had a digital one from the big car days.