Tween [AS2]

programming flash
1 

this.mainpage_btn_mc.onRollOver = function(){
	mainpage_btn_mc.gotoAndPlay("over");
new Tween(belowmain_mc, "_y", None.easeNone, 79, 110, 12, false);
}
this.mainpage_btn_mc.onRollOut = function(){
	mainpage_btn_mc.gotoAndPlay("out");
new Tween(belowmain_mc, "_y", None.easeNone, 100, 79, 12, false);
}

I have the above code so when a button ‘mainpage_btn_mc’ is rolled over another mc ‘belowmain_mc’ tweens down, and on rollout it tweens back up.

new Tween(belowmain_mc, "_y", None.easeNone, [COLOR="Red"]100[/COLOR], 79, 12, false);

what i am wanting to do is set the start position to the current y value instead of a y coordinate, what would i need to achieve this?

-ollie