The durometer I use is a very cheap Chinese one. Something like $15 or so. It's certainly not very accurate, and nothing scientifcally usable - but it indeed generates somewhat reproducable results.
The problem with those measurements is that you'd need a well defined block of the tire rubber. And a well defined force for a well defined time. But tire knobs on mountainbike tires are particularly bad for measuring the shore value, because they're too small, have ridges, grooves, sipes, and what else is there.
I've tried to measure the tires available to me, and while there are a couple of inconsistencies, other results seem to match the official values pretty good.
fotos.mtb-news.de/p/2088921 (my comments are in German, but you'll understand the important parts)
Wolfgang Arenz has developed the BCC compound for Continental years ago, then went on to Specialized, who now have their Gripton compound, and finally ended up at Schwalbe who recently introduced their Addix compound. At least there seems to be a pattern here... I wonder where he'll end up next. Maxxis would be a good choice, they could need a temperature resistant compound...