TaxPackagingQty to SalesLineGrid

Hello,
I want to show InventTable.TaxPackagingQty field in SalesTable form SalesLineGrid . What I did:
I created new data source InventTable in SalesTable form and put the TaxPackagingQty field to SalesLineGrid in that form. http://i50.tinypic.com/1zz2nmw.jpg. I put couple methods under InventTable data source so new SalesLine wouldn't write to InventTable and wouldn't throw errors:

1)
public boolean validateWrite()
{
   boolean ret;
   ;
   //ret = super();
   ret = true;
   return ret;
}

2)
public void write()
{
 //  super();
}

And I made InventTable data source non editable like this: http://i48.tinypic.com/33c6l8z.jpg.

The problem is when I creat new Sales lines, TaxPackagingQty  field shows zeros. But when I close and open again SalesTable form, TaxPackagingQty  field shows correct values. What did I wrong? Or maybe there is a better way to show TaxPackagingQty in SalesLineGrid?