PO Revision Variance - PO Approval Workflow Position Hierarchy

Hello,

We have a Position Hierarchy PO Approval Workflow tied to document total.  It works perfectly on the creation of the PO and initial approval.  After the supplier comes back with price changes, our buyers then go back into the PO and change unit prices.  This triggers a PO revision and as long as the PO document total is still above the first threshold it get sent through the PO approval workflow position hierarchy.

How do we change this so it just looks at the variance between the original document and the revised document?  If the change is for only $10, even if the PO total is a million dollars we don't want it to have to get sent through the approval process again.  Hope this makes sense, if not please ask questions and I will do my best to clarify further.