[OXO cube]

I would take the overall tyre width as measurement personally - all tyres will need a rounding on the edge for consistent performance IMO.
Using a 13mm wide tyre and rounding it so the contact area is 12mm does not make it a 12mm tyre  .... just my tuppence worth 

agreed 
ppl usually buy max width wall to wall 12mm tyres , if 12mm is the stated rule 
but you are the rule maker , so all good