Another volume slider question

Hi guys,
I made the slider using the tutorial, which works great.

However, I want to make the slider so when the sound is started, the slider would start out in the middle of the slide bar, therefore 50% of the volume.
from there the person can either make it louder or decrease the volume.

how can i accomplish this.

the following is the code in the slider clip:

this.ratio = 0;
dragger.onPress = function() {
this.startDrag(true, 0, 0, line._width, 0);
this.onEnterFrame = function() {
ratio = Math.round(this._x*100/line._width);
_root.volume = ratio;

};

};
dragger.onRelease = dragger.onreleaseOutside=stopDrag;

thanks in advance