// When you are in a for loop, create new items and and try to measure the height of an item.
// And position items vertically.You need to remember the previous item, the fastest way to do this is to have a runningY var declared outside the for loop with an initial value of zero.
//See comments in code for further explanation.

var runningY:Number = 0;
			
for (var index:int = 0; index < 4 ; index++) {

	var _textheader:TextField = new TextField( );
	_textheader.text = "Text";
	_textheader.autoSize = TextFieldAutoSize.LEFT;
	_textheader.x = _background.x - _background.width / 2 + TOP_LEVEL_OFFSET_X;
	// First iteration  runningY = 0;
	// Second iteration runningY = 0 + textHeader.height +2; textHeader.height = 20
	// Third iteration  runningY = 22 + textHeader.height +2; textHeader.height = 40 -- mutiple lines of text
	// Fourth iteration runningY = 64 + textHeader.height +2; textHeader.height = 20
	// Fifth iteration  runningY = 86
	_textheader.y = runningY;
	runningY += _textheader.height + 2; // textheader = 20
}

Spacing items vertically (y axis)

ActionScript 3