Syncing with + or - a certain amount, rather than syncing to match.

I want to add the same amount of contrast / white balance change. 

As in, + 4 for all contrast, but not to sync.  So an images with 23 contrast will  + 4, and an image with 0 contrast will  + 4.  Same with white balance- plus or minus a certain amount of Kelvin. 

Thanks.