Drag, Drop and Constraining - HTML5 Canvas

OK, I have the drag & drop sorted, and now need some help with the snap alignment.

The snap is working, BUT I need to get the alignment of the draggable correct with the drop target.  At the moment the dropped symbol is sitting slightly below centre of the hitBox.  You can see this with the hitbox (large purple rectangle - for testing semi transparent) on the target and the smaller square purple symbol overlayed near the pupil - the dot mc on the droppable.

Rather than adjust the transform points of the drop and target symbols, how can I change the code to play with the offset? 

I can't change the transform points of the symbols, as I need to rotate them later after the drop on a common centre point (the centre of the lens and the middle of the holding frame (centered on the pupil)).

//Drag and Drop lens onto trial frame behaviours
createjs.Touch.enable(stage);

var Lenses = [this.ConcaveSphereLens, this.ConvexSphereLens, this.ConcaveCylinderLens, this.ConvexCylinderLens, this.PrismLens, this.AccessoryLens];

	// Apply the same code for each lens MovieClip symbol in the array Lenses with a For Loop
for(var i = 0; i<Lenses.length; i++){	
	Lenses[i].on("mousedown", onMouseDown.bind(this));
	Lenses[i].on("pressmove", onMouseMove.bind(this));
	Lenses[i].on("pressup", onMouseUp.bind(this));
	Lenses[i].LFLensHolder = this.LFLensHolder;
	Lenses[i].originX = Lenses[i].x;
	Lenses[i].originY = Lenses[i].y;
}

function onMouseDown(evt){
	var item = evt.currentTarget;
	item.offset = {x:0, y:0};
	var pt = item.parent.globalToLocal(evt.stageX, evt.stageY);
	item.offset.x = pt.x - item.x;
	item.offset.y = pt.y - item.y;
	item.drag = true;
}


function onMouseUp(evt){
	var item = evt.currentTarget;
	item.drag = false;
	var pt = item.localToLocal(item.dot.x, item.dot.y, item.LFLensHolder.hitBox);
	if(item.LFLensHolder.hitBox.hitTest(pt.x, pt.y) ){
		item.x = item.LFLensHolder.x;
		item.y = item.LFLensHolder.y;
	}
}

function onMouseMove(evt){
	var item = evt.currentTarget;
	if (item.drag){
		var pt = item.parent.globalToLocal(evt.stageX, evt.stageY);
		item.x = pt.x - item.offset.x;
		item.y = pt.y - item.offset.y;
	}
}