I'm having problems with my ramdom rock generater. It makes all the rocks i need but, when I want to center them to get an even rotation it goes out of wack. how can i center the rocks to get the even rotation?

just copy past code in frame to work it


Code:
//total rocks at one time
totalrock = 10;
//rock id number
ir = 0;
//interval that puts out rocks
rocky = setInterval(this, "rockgen", 1000);

function rockgen() {
	var done = false;
	if (ir>=10) {
		ir = 1;
	}
	while (ir<totalrock && !done) {
		ir++;
		if (this["rock"+ir] == null) {
			done = true;
		//create rock
			this.createEmptyMovieClip("rock"+ir, ir);
		//create rock drawing
			this["rock"+ir].createEmptyMovieClip("p", 1);
			
		//Rock drawing
			this["rock"+ir].p.beginFill(0x333333, 100);
			this["rock"+ir].p.lineStyle(3, 0x666666, 100);
			//point x
			this["rock"+ir].p.lineTo(0, 0);
			//point 1
			this["rock"+ir].p.lineTo(ran()+20, ran()+10);
			//point 2
			this["rock"+ir].p.lineTo(ran()+30, ran()+30);
			//point 3
			this["rock"+ir].p.lineTo(ran()+20, ran()+50);
			//point 4
			this["rock"+ir].p.lineTo(0, ran()+60);
			//point 5
			this["rock"+ir].p.lineTo((ran()+20)*-1, ran()+50);
			//point 6 middle
			this["rock"+ir].p.lineTo((ran()+30)*-1, ran()+30);
			//point 7
			this["rock"+ir].p.lineTo((ran()+20)*-1, ran()+10);
			//point x
			this["rock"+ir].p.lineTo(0, 0);
		//Rock drawing end
		
			//center code
			this["rock"+ir].p._x = this["rock"+ir].p._width/2;
			this["rock"+ir].p._y = this["rock"+ir].p._height/2;
			this["rock"+ir]._x = int(Math.random()*400);
			
			//rotation cw or ccw
			var chosse = int(Math.random()*2);
			switch (chosse) {
			case 0 :
				this["rock"+ir].rospeed = int(Math.random()*20);
				break;
			case 1 :
				this["rock"+ir].rospeed = int(Math.random()*20)*-1;
				break;
			}
			
			this["rock"+ir].onEnterFrame = function() {
				var t = this;
				t._y += 20;
				t._rotation += t.rospeed;
				if (t._y>Stage.width) {
					removeMovieClip(t);
				}
			};
		}
	}
}
//retruns a random number
function ran() {
	return int(Math.random()*5);
}
this is the code i use to center the rock

Code:
//center code
			this["rock"+ir].p._x = this["rock"+ir].p._width/2;
			this["rock"+ir].p._y = this["rock"+ir].p._height/2;