XL reporter expert question

I am trying to produce a stock item Quantity consumption report with XLR and I am choosing the Sales AR and Sales Credit Memo type of transactions. The problem I find is in choosing a light dimention field that will give me the Item Stock Quantity per invoice line. I have tried the INV_Qty field but it did not give me the right result. Any ideas where am I going wrong? I have also tried the SO_Quantity field and this did not work either.

Thank you for any tips?

Robert