Withhold amount excl VAT when VAT is applicable

Summary

An amount has to be withheld on all invoices that mention VAT. The percentage that needs to be withheld is 6% of the paid amount excl. VAT

Content

Hi guys,

Anyone knows how to setup the following?

In Kenya an amount has to be withheld on all invoices that mention VAT (where VAT is applicable and >0%). The percentage that needs to be withheld is 6% of the paid amount excl. VAT.

I have done similar setup, but where VAT amount had to be withheld instead of amount excl VAT. Any suggestions? I tried working with Tax Applicability rule but so far no luck, as it is calculating withholding over VAT instead of invoice amount excl VAT.