Currently we can use variable only in the Property Checkpoints. I propose to extend this feature to the Object Checkpoints, too.
Maybe this will help. It's a previous forum post that shows how to update a stored value from a script. You could update the stored value at the top of the test each time.
http://community.smartbear.com/t5/General-Discussions/Variables-in-stored-objects-value/td-p/47856