Add an additional discount based on the total line value for a specific item type

Summary:

An additional discount is required if a specific "class" (UDA on item master) is ordered for over $4500 on the order

Content (required):

We have already implemented Pricing at Account level and discount based on Item Master UDA, Customer Master Account Attribute and Order Type. Cumulative discount is also enabled on the Order.

We are trying to add an additional 30% discount on the order when a specific class of item is ordered for over $4500. We have added the Matrix rule for Order Total under the Pricing Term Adjustments, however when we use the "Compare to Attribute" as Total.TotalAmount or SalesOrderTotal.TotalAmount, the sales order is throwing an exception on save / re-price.