How can I multiply a merge field in Word?

I have a mail merge document that outputs values from another piece of software (XERO). I need to multiply one of the merge field values to show a different value on the actual document.

e.g The merge field outputs 10.00 and I want to multiple the output value by
0.15 to display and print the value as 1.50

Please see my field below:

{ = {MERGEFIELD InvoiceAmountDue \* MERGEFORMAT}*0.15 }

When update the field it returns  !Syntax Error, <<

Can anyone please help?

Kind regards