O3D moving cuts by mouse - medical data

[Xrew]

Hi,
i haven't solved my problem yet.
I have made some progress, but it's still not OK. When i click, i get
intersection point of mouse and object in 3D. Then i calculate normal
of cut, which i collapse to screen. So i get two vectors
Normal of cut and mouse vector on screen. Then i calculate scalar
product (dot) of these vectors and it should be a direction, where
should i move cut. Cut is moving to right direction, but it all depend
on zoom of scene. If zoom is 1 everthing is ok...but, if it's another,
the cuts are moving to quick or too slow...(mouse is quickier or
slower)..I think, it can be resolved by converting ratio between
screen distance and scene distance...but i dont't know how...
I'm attaching screenshot of my app in hope to help you with
understanding to my problem.
Thanks a lot.
http://www.image-share.com/upload/580/111.png