The track adjustment is correct when the track has 1-1/2-2 inches of sag measured at a point 1/2 way between the front idler and rear sprocket centers.
The book shows a stright edge laying on top of the track as a reference
I have a top idler in the center. Is it capable of that much sag in the center?
Why do some D25Us have a top roller and some not?
The carrier roller started out as an optional attachment on the D2
Well I know this does not exactly follow the book but the way we used to set them (and we ran them for thousands of hours) was to put a bar on the carrier roller stanchion and pry the rail up. We expected about 1 and 1/2 in raise.
And that is the way I set my D2 today.