Actionscript 3 :: Flash - Center An Object Around A Point After Scale?

I've created an mc which functions as my map and controls to go with that which allow the map to be moved around and key points navigated to and zoomed in on at varying zoom levels.I'm using scaleX and scaleY to scale the map mc and a list of x and y positions to correctly position the map for each key point.When I want to go to a certain area I perform this calculation (offsetX and offsetY are the center of the screen):

newX = posX * scale + offsetX;
newY = posY * scale + offsetY;

Then I tween the position and scale of the map to smoothly scale and move the map to the correct position:

var tween = new TweenLite(_background, EASING_SPEED, {x:newX, y:newY,scaleX:scale.getScale(),scaleY:scale,ease:EASING_TYPE,onComplete:moveToComplete,onCompleteParams:[room]});

I now need to implement a zoom in / out function and this is what I'm struggling with. The zoom should use the center of the screen as the reg point for the scale, so I've been trying something along the lines of the following to get the correct positioning:

var newX = offsetX - (offsetX - _background.x) * scale;
var newY = offsetY - (offsetY - _background.y) * scale;

So in my mind this gets the distance from the current position of the map relative to the center point of the screen (offsetX, offsetY) then scales that distance by the new scale. However, this is clearly wrong because it's not working and the positioning of the map is wrong.I've also tried using a transform matrix to get the correct values but I know even less about them and not got the right results.

function scale(target:MovieClip, center:Point, scale:Number):Point {
var m:Matrix = new Matrix();
m.translate(-center.x, -center.y);//move the center into (0,0)

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