ActionScript 3.0 :: Rotating Around Matrix Point To Specific Angle

I have a slider which I want to use to rotate an image around it's center. This normally is easy but my problem is the clip is made dynamically and the size changes so I can't use the default registration point and I need to assign the center point before I rotate it. I found the rotateAroundExternalPoint function which rotates around a center point fine. How to get it to rotate to a certain angle and then have it stop at that angle. The rotateAroundExternalPoint increments the rotation rather then allowing me to set the angle to rotate to. Is there anyway that I can set what angle to rotate my clip to? It right now increments the rotation by the x coordinate of my slider bar (slider_position.x).

Code:
import flash.geom.Matrix;
import fl.motion.*;
var mat:Matrix = clip.transform.matrix;
addEventListener(Event.ENTER_FRAME,EnterFrame3);
function EnterFrame3(event:Event) {
var center_point_x = clip.height / 2
var center_point_y = clip.width / 2 //
MatrixTransformer.rotateAroundExternalPoint(mat,center_point_x,center_point_y,slider_position.x);
clip.transform.matrix = mat;
}