Withholding Tax calculation based on Invoice Threshold

Dear OSC,

We have a requirement to calculate Withholding tax based on invoice threshold for El Salvador. i.e if the invoice is less than 100 USD, no WHT to be deducted, if the amount is more than 100 then 1% WHT to be deducted.  Please share your inputs If anyone had similar requirement and resolved the issue.

Withholding Tax code Setup like below;

Rate structure : Amount Range

Period basis    :  Invoice             

Amount Basis' : Gross Amount

Tax Rates   :     0 - 100     0

                     100 -            1%

1) The withholding will not be applied for invoices below $100 - that is correct

2) Invoices amount above $100 . i.e. 200 then the withholding tax is calculated as $1 (1% of (200-100)), but expected is $2 (1% of invoice gross amount $200)