TaxPackagingQty to SalesLineGrid

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. I put couple methods under InventTable data source so new SalesLine wouldn’t write to InventTable and wouldn’t throw errors:

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

public void write()
// super();

And I made InventTable data source non editable like this:

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?

If possible add another field to the salesLine and pull the information from the inventTable and update this field

Else use a display method, look into itemName in salesLine how it has been used, \Data Dictionary\Tables\SalesLine\Methods\itemName

The first way had the same problem as I described. The second way with method example worked fine. Thank you very much!