Withholding tax calculation in Cash Journal

Dear Gurus,

I have extended Withholding tax activated and created tax codes for posting tax during payment.

When I post a vendor invoice for e.g I get the following entry:

Dr.Expense    900         USD

Dr.Input tax    100         USD

                Cr.Vendor              1000   USD

The vendor is subject to withholding tax of .05% that is 4.5 USD for this transaction and this is the value I get when trying to post a manual outgoing payment in Withholding tax tab page.

But, when trying to post vendor outgoing payment using cash journal (transaction FBCJ) the 0.5% is calculated on the 1000 USD as a base amount not the 900 USD and hence the withholding tax amount is calculated as 5 not 4.5 USD. What am I missing here?! Can this be changed?

Thanks for help.