Enter Money In transactions Net of GST

Using Items for Money In for transactions with GST. Settings are set to enter net figures. When entering net amount, GST is calculated as if amount entered is a Gross Figure. The only way to make it work is to change settings of item to have no tax code, then after entering amount in transaction, select GST and the figures will be grossed up. Invoices show net plus GST with a Gross Total. Why cant this be done in the receive money window?