Help on adjusting the speed of easing on mousemove!?

hey all,

i have the simple easing code:

bg_mc.onEnterFrame=function(){
speed = 30;

this._x+=(_root._xmouse-this._x)/speed;
this._y+=(_root._ymouse-this._y)/speed;
}

// ok this work fine on a small movieclip
but when my movieclip is 5500 x 5500 px big;
the small movements are to fast…
so what is a smart Math way to calculate if the mouse is moved slowly the speed is also slowly if the user moves fast the speed is very fast