Getting the current position of a MC

Hello again everyone.

(Bear with me, I’m a little bit angry and frustrated right now…)

Working on an mp3 player, I realised that I stumbled upon a problem that I ALWAYS have, but there seem to be very very little documentation about.

I have a movie clip - every time a button is pressed, I want to move the movieclip 11 pixels up. - see it’s a masked menu. I want to do this with the Tween class.

But since I’m an idiot, i can’t for the life of me figure out how to get the current position of the mc, and thereby not making it do the same motion every time the button is pressed.
As you may well know- the tween class uses a start position/value and an end position/value:

new Tween(some_mc, "_y", Strong.easeOut, 'current position', -11, 20, false);

That’s my dream Tween there… But why can’t you do that? Grrrrrrrrr…

What I need is two constant values - so no matter where the mc is, the tween will always subtract 11 from the current position.

I’ve tried this, but it doesn’t work:

 var yPos = this._y;
var yDest = (this._y - 11);
this.up_btn.onRelease = function() {
    new Tween(lib_mc, "_y", Strong.easeOut, yPos, yDest, 40, false);

Am I going about this in the right way?

And I wonder why I can’t find anything about this problem. I mean, it must be something everyone experiences. Or at least it seems so to me, because I encounter this problem all the time. Maybe I’m just a n00b at searching the net aswell? Hehe!