Custom tax rounding method (with rules?)

I am aware that you can choose the method to round your tax here:

/admin/commerce/config/taxes/types/sales-tax/edit

However my client requires an option to round down (drop all numbers after the decimal point).

Is there some way to implement this? I tried to manipulate this rule, but no success:
/admin/config/workflow/rules/reaction/manage/commerce_tax_type_sales_tax

Thank you very much for any assistance that could be given.