MV_X = (BALL._x - (PDL._x + PDL._width/ 2)) * 8 / PDL._width;

import flash.geom.Point;
// 原点(0, 0):
// var p_0:Point = new Point(0, 0);
// 変更前(10, 10):
var p_1:Point = new Point(10, 10);
// var nDistance:Number = Point.distance(p_0, p_1);  // 長さ
var nDistance:Number = p_1.length;  // 長さ
trace(nDistance);  // 出力: 14.142135623731 = 10√2
// 変更後(5, 15):
var p_2:Point = new Point(5, 15);
p_2.normalize(nDistance);  // 長さを同じに
// trace([p_2.x, p_2.y, p_2.y/p_2.x, Point.distance(p_0, p_2)]);
trace([p_2.x, p_2.y, p_2.y/p_2.x, p_2.length]);
// 出力: 4.47213595499958,13.4164078649987,3,14.142135623731
// xy成分: (4.47213595499958,13.4164078649987)
// y/x = 15/5 = 3
// 長さ: 14.142135623731 = 10√2

(0,0) と現在のポイント間の線のセグメントを設定された長さに拡大・縮小します。

// 原点(0, 0):
// var p_0:Object = xPoint(0, 0);
// 変更前(10, 10):
var p_1:Object = xPoint(10, 10);
// var nDistance:Number = xDistance(p_0, p_1);
var nDistance:Number = xLength(p_1);
trace(nDistance);
// 変更後(5, 15):
var p_2:Object = xPoint(5, 15);
p_2 = xNormalize(p_2, nDistance);  // 長さを同じに
// trace([p_2.x, p_2.y, p_2.y/p_2.x, xDistance(p_0, p_2)]);
trace([p_2.x, p_2.y, p_2.y/p_2.x, xLength(p_2)]);
function xPoint(nX:Number, nY:Number):Object {
    return {x:nX, y:nY};
}
/*
function xDistance(oStartPoint:Object, oEndPoint:Object):Number {
    var nDistanceX:Number = oEndPoint.x-oStartPoint.x;
    var nDistanceY:Number = oEndPoint.y-oStartPoint.y;
    return Math.sqrt(nDistanceX*nDistanceX+nDistanceY*nDistanceY);
}
*/
function xLength(oPoint:Object):Number {
    var nDistanceX:Number = oPoint.x;
    var nDistanceY:Number = oPoint.y;
    return Math.sqrt(nDistanceX*nDistanceX+nDistanceY*nDistanceY);
}
function xNormalize(oPoint:Object, nLength:Number):Object {
    // var nOriginalLength:Number = xDistance({x:0, y:0}, oPoint);
    var nOriginalLength:Number = xLength(oPoint);
    var nNormalizeRatio:Number = nLength/nOriginalLength;
    return {x:oPoint.x*nNormalizeRatio, y:oPoint.y*nNormalizeRatio};
}