Are you in the insurance business? My understanding of the 80% clause is that if you insure your home for 500K , your replacement value (as determined by the insurance company) can be as much as 500k/0.8= 625k and your under-insurance clause will not be in effect. So if you have a 250k loss they will pay the entire 250k and not be reduced. max amount they will pay is 500k.