I'm assembling a brand new MTB and have purchased a (setback) Thomson seatpost and a Terry Butterfly saddle. What is the fore/aft adjustment range for this combination?

The Butterfly's rails are marked like this:
STOP> |....i....|....i....|....i....|....i....| <STOP

The ruler portion is 40mm long. Between the tips of the STOP arrows is 47mm. Unfortunately, the Thomson rail clamp is 41mm long.

Does this combination really have only 6mm of official adjustment range? Or are the Butterfly's markings meant to indicate the range for the center of a (much shorter) clamp?

Right now I tentatively have the saddle pushed back as far as possible with the front edge of the clamp right up to a bend in the rails. The middle of the clamp is still within the 40mm interval. However, I don't know if this will be back far enough. My rough measurements show that the saddle on my old MTB is even further behind the bottom bracket - but that saddle might never have been properly adjusted.