with all scrollbar draggers (whether they are vertical or
horizontal or something else) and all objects that will be scrolled
(whether they are vertical or horizontal or something else) , you
can assume that there will be 2 extreme positions of the scrollbar
dragger and 2 extreme positions of the object being controlled and
you can assume a linear equation will be used to give the position
of the object depending upon the position of the scrollbar dragger.

so, in your situation of a vertical dragger and vertical
movieclip:

mc._y=a*dragger._y+b; // where you solve for a and b using
the two equations that result from knowing the 2 extreme dragger
positions and the two corresponding extreme movieclip
positions.