I am trying to make a sliding menu whose speed is responsive to _xmouse (slow at the center and fast at the sides…) does it need to exist in two pieces? the problem is that it seems impossible to define 0(zero) as anything but just that… does this make sense to anyone?

if you are trying to make the center 0 but the 0 is located at the top left you could use:
center = this._width/2;
then calculate speed by finding the distance between the mouse and center ie
speed = abs(center-_xmouse)/10;//less than 10 equals faster
hope that helps,

thanks for your reply - I will try to use that somehow

Did you read this?