object positioning

I need to be able to change the position of an object (such as a box) within a window relative to it’s current position. e.g. move a box
the best way I’ve found to do this is;

  tell window "thewindow" set position of box "thebox" to {(item 1 of (get position of box "thebox") as integer), ((item 2 of (get position of box "thebox") as integer) + 100)}

This works fine except that this way i can only set the the position of the box relative to the bottom left corner which is not ideal in every circumstance.
Is there a way I can specify the origin? (i assume this is possible here because it is in interface builder).
many thanks