There is a lot of flexability in the formulas. I would base it on the Carbonate amount, and then just balance the solution to pH of 6+-. That will work well without any difficulty.

Since the oxalic acid is always in some form of hydration, it's hard to know how much you are really putting in, but it is a simple matter to put in an amount and then test for pH and add more to obtain the correct pH. Ultimately, that is all that is important anyway, assuming you are mixing a high enough concentration of PO in the first place, which the amounts you provided indicate you are.

I think when I got my container from The Chemistry Store it wasn't as wet as it is now, so I suspect that the hydration has changed with time. Pennsylvania humidity will do that to you...