This is a theorical problem.
What you should known first is; are you are able to take off with your A-320from such an airport(8500 p.a, 28 degrees...) at MTOW...i doubt
That's why i would turn your question upside down:
What is my Max RTOW (regulated take off weight) for this 8500 p.a Airport, 28 degrees..(in order to fullfill the minimum climb gradient on one engine (2.4 % for the 2nd segment,1.2% for the 3rd segment))?
Performance Table or PC for this particular airport will tell you taking everything in consideration..obstacle....