Ian, I think the measurement is taken at the front of the tyre itself, thus largely removing the rim diameter issue.
From memory, the alignment machine works in degrees of toe-in, each side being measured separately. A manual input of the tyre diameter translates the degrees to mm (or inches) of toe-in at the front of the tyre.
I think that the idea of mm or inches of toe-in came from the old days when a manufacturer was fairly sure that their standard wheel/tyre combo was going to be used. With all the options nowadays, perhaps the degree setting is the right one to go with ?
Kind Regards,
Peter D.