Basically it would be something like this.
while SliderBar is in Touch
set SliderBar.X to Touch.X
Every Tick set Zoom to SliderBar.X
You may even want to do every 0.2 seconds or 0.5 instead of every Tick.
This will give it a more fluid effect and not be all twitchy.
Where "Zoom" would be the global variable that controls the amount you are zoomed in by. Does that make sense?
My brain is a little fried. Been sick all week.