Sheap, Try setting the finder in the centre of the holder/mount, (eg; screws about equal). Now, If you look along the length of the tube from the back, does the centre line of the finder, line up with the centre of the tube? If not, then loosen the 2 mount screws & try to adjust, so it does.
You may have to elongate the mounting holes??????? a little..
A pic of the mount & how it's sitting on the tube,(pref from back/tube centre) would help here please, if poss..
Hard to visualise..
HTH..
L.