paul - someone has probably already answered this, but i mark off the
mm measurement onto a 1/4 inch wide strip of cardboard or magazine
insert, cut it at the mark, tape it & slip it onto my ring mandrel
for the standardized ring size, which has sizes broken into 1/4s.
from then on i work on the mandrel at that size. for customers
sending their ring finger measurement by mail i have them cut a 1/4
inch wide strip from an old business card or a torn-out magazine
insert, roll it up once, wrap it around the largest knuckle on the
proper finger so that there are 2 thickness on top & stick a pin
through both. they then send that to me; the vagaries & inaccuracies
of measuring rulers or tapes that people have on hand, misremembered
sizes, etc., are all eliminated by this method that treats the length
as a cipher rather than a possibly inaccurate measurement.
good luck -