How to calculate the amount of buffer needed to keep a solution at stable pH?

I have a solution (1.5L) that while it degrades changes its pH from 7 to about 3.5. I need to add a phosphate buffer {[(Na2HPO4 5.7g)+(KH2PO4 3.6g)]/L} to keep it stable at pH 7. How do I calculate the amount of buffer needed?How to calculate the amount of buffer needed to keep a solution at stable pH?
you can't, you can only determine it empirically. A buffer will only control the pH at up to1pH unit each side of a pKa value. Outside of that, it is not buffered.