Withholding tax

hi to all,

I have a little doubt, I need to create a withholding tax, but the tax rate must be applied only to the  90% of the gross amount per invoice.

Actually, we just calculate the 10% of the max amount and define the tax rate area like this (let's assume the max amount is 10000, and the 10% is 1000):

from    to           Rate

0         1000      0

1000                 10

Any tips will be welcome. Thanks in advanced