// Your everyday absolute tween that moves movieclip_mc
// to 100 on the x axis, taking one second to do so
TweenMax.to(movieclip_mc, 1, {x:100});
 
// A relative tween that moves movieclip_mc to a
// position on the x axis 100 units/pixels greater
// than movieclip_mc's current position
TweenMax.to(movieclip_mc, 1, {x:"100"});
 
// An absolute tween like the first, but passing in
// the new value with a variable
TweenMax.to(movieclip_mc, 1, {x:newX});
 
// A tween that casts the variable as a string,
// thus making it a relative tween
TweenMax.to(movieclip_mc, 1, {x:String(newX)});

http://patrickmcd.com/2009/08/17/tweening-to-a-relative-position-using-a-variable-in-tweenmax/

If you want to tween an object relatively, but the new value changes and needs to be a variable, just cast it as a string.

Tweening to a Relative Position using a Variable in TweenMax

animation

ActionScript 3