Is it possible to post an adjustment to OwnPostAdj using FinanceFunctionType.Calculate?

To Henning's point, you need to make sure your calculation is only running on the C#OwnerPostAdj member. So (I'm guessing) you will want to wrap your calculation in an If statement to ensure it only calculates on the OwnerPostAdj consol member, e.g:

If api.Pov.Cons.MemberId = DimConstants.OwnerPostAdj

End If

Also, looking at your error message, I would take a guess and say you shouldn't need to include entity or Consolidation members in your destination script since these will be determined by the Data unit being processed. 

Thanks