I got an email today that Festool will be offering some tools calibrated in Imperial (inch) measurements
http://blog.festoolusa.com/2016/06/20/festool-will-begin-offering-imperial-scale-tools/
I see that the Domino machines are not listed - but on mine I use an aftermarket offering from Seneca
http://www.senecawoodworking.com/search?type=product&q=imperial
I would love festool to start making their own imperial but I did pick up an aftermarket one myself not too long ago from another site.
It’s a great there for here in the states since it makes adjusting the boards so much easier. Festool should just have it be default imperial for the USA dominos.
I didn’t go with the Seneca one, but bought a 3D printed one and it works great.
https://toolcurve.com/product/festool-domino-500-imperial-thickness-gauge/