I don't have a RZ67 to look at, and the only illustration I have available lacks the colour necessary to pick out which colour curve relates to the 90mm, but I'm not clear that any of the curves "flatline".A corollary question, re “read the distance and set the ring”… I presume you are talking of reading the distance of the graph on the side? On the 90mm the graph flatlines after a meter or two… I presume after that you guesstimate?
Are you sure you are using the Distance Scale in the manner intended?
FWIW, I used to use my scale focusing experience when I did this with my RB lenses with floating elements, rather than relying on the Distance Scale.