Booking Field - Avg Nightly Rate Percentage

Not sure how this would be implemented but I send guests that have open adjacent nights a discount offer.  The issue is that when I offer 15% or 30% I have to do all of the calculations manually to update the reservation with the associated discount.  It would be nice if I could send an extra night offer with a booking field that does the additional night calculation automatically (calculates the 15% off the ADR + taxes/fees) and just puts in the total amount extra it would cost (ie. $350).