Ease to effect

I’ve been using the ease to effect for quite some time and today I just have had my limit. Because everytime I try to use it I always have to enter in X, Y coordinates that make no sense and I have to guess and check till its perfect and I want to know why. I have my one object with this on it :
onClipEvent (load) {
_global.tarX = 50;
_global.tarY = 146;
}
onClipEvent (enterFrame) {
this._x += (tarX-_x)/3;
this._y += (tarY-_y)/3;
}
Now its current location on the root level is 0,0 along with its location in its own movie clip 0,0. Now why does it now go to 50, 146 like it should?