AS 3 Tween Class help

I could really use some help. I'm using AS 3 and am creating a simple vertical menu where 6 buttons reside. When any of the buttons is clicked it takes you to specific frame label. On each frame label I have some actionscript that once arrived at will animate a small triangle along the y axis to rest next to the button the user has just clicked. I am trying to use the Tween Class to aminate the small triangle. Here is what I have so far:

import fl.transitions.Tween;
import fl.transitions.easing.*;

var arrowTween:Tween = new Tween(arrow_mc, "y", Regular.easeOut, 20, 80, 2, true);

The var created assigns the triangle a new end position. Here's where I need help. In the example above the 20 is the starting position and what I need is something that says whatever the current (this) position value is move to the desired end position value, which is 80 in the example.

I'm up late trying to figure this one out. Any help or ideas to lead me in the right direction is greatly appreciated.