Jim, If I'm understanding you correctly here, they've changed thier method of calculation on these "catch ups;" originally charging 13% on the DIFFERENCE, and now charging 13% on the TOTAL. The allowance(s) already included a 13% upcharge, and now trying to charge 13% on the total allowance which already included 13%.
Jim, I have to say I stand by my initial thought of argueing this. I'd first discuss it with the builder and, if nothing were settled, my attorney.