No, this doesn’t quite work! I set up “US Letter” as a default according to your instructions.
But when I create a new score with multiple players, Dorico duly formats the Full Score in US Letter, but each of the Player individual parts are automatically formatted by Dorico for A4!
Sibelius has always done exactly the same thing. Create a new score, set the Layout to US Letter, yet the individual parts get automatically formatted to A4.
The only thing that a Dorico user could do at this point is for the user to go to the Setup mode, choose the Layout settings for each individual Player in turn, and manually change each to US Letter, and then go and check to see how things have been shuffled around on the pages as a result of the reformatting.
This is a real problem for a composer or copyist. A person could carefully create a full score in US Letter and easily overlook that Dorico has formatted pages for individual parts for the wrong paper size, A4. Then those parts get sent off to the USA musicians as PDF files. Everybody prints out their parts on their local USA printers and wonders why their parts all print out smaller than they are supposed to (making them harder to read on the music stand), formatted too tall and too narrow on the pages of paper. I have seen this happen with Sibelius scores time and time again – the full score is formatted to the correct dimensions but the individual parts are formatted wrong because what Sibelius did with A4 was not at all obvious to the composer.
I would have hoped that after all these years I wouldn’t have to deal with this anymore, because Dorico is designed not to be burdened with legacy problems.