Congratulations. :-) I knew there was some sort of hyberbolic relation in there, but I give up easy. :-)

In my defense, I was close enough for government work, I had

 $sc[$i]->xviewMoveto( ($location /$initial_width );
# where I needed
 $sc[$i]->xviewMoveto( ($location - 400/2)/$initial_width );
[download]

Of course, your work on zooming gets you an A++.

This definitely is a snippet to save, since it is a general purpose formula for centering a canvas item in a scrolled window.