Alpha Slider

Can somebody help me fix this slider. It works but something is off with my math and it doesn’t see the full length of the slider. Resulting in only half of the slider actually changing the alpha of the moviceclip.

My slider is 282 pixels wide. Here is the code…

[AS]
var ratio:Number = 0;
dragger.onPress = function()
{
this.startDrag(true, 232, 800.2, 494.1, 800.2);
this.onMouseMove = function()
{
ratio = Math.round(this._x);
};
};
dragger.onRelease = dragger.onReleaseOutside=function ()
{
this.stopDrag();
this.onMouseMove = undefined;
};
this.onEnterFrame = function()
{
bigscan2._alpha = -100+((dragger._x-175)/1.40);
};
[/AS]

Here is a link to the FLA file if you want to see what I mean. Thank you for any help on this…

Sincerely,
Greg