var clipTotalWidth:Number = 400;
var clipCenter:Number = 200;
var posX:Number = (put here your _x property);
var absoluteDistance:Number = 0;
var distance:Number = 0;

if (posX > clipCenter && posX <= clipTotalWidth) {
	absoluteDistance = posX - clipCenter;
	distance = (absoluteDistance / clipCenter);
	
} else if  (posX < clipCenter && posX >= 0) {
	absoluteDistance = clipCenter - posX;
	distance = (absoluteDistance / clipCenter);
	
} else if (posX==clipCenter) {
	distance = 0;
}