Sorry -- after I posted I noticed the adjustable in your post. The adjustment when John is done with the RRA is just to add or take out a little of the "grasshopper bend" he puts in the trigger spring.
I had Holliger tune a RRA and then installed that in my AR10. John likes to have a little more weight on the RRA if it's going into a AR10 than an AR15, but you can lighten his first stage spring treatment a little if you like.
http://www.whiteoakarmament.com/ is the place to order, probably about $115. You can get a RRA trigger here for about $90 shipped, but it's hit or miss. With John's, you know it'll be smooth.
Hated the Armalite that came in my AR10(T), but the previous owner (or whoever) had butchered it. Had to trash it (John couldn't do anything with it)