Incorrect withholding basis amount

Hi Users

We have two withholding classes  one 00001 with  10 % and other  00002  with 2%      both are   'Apply Net'

when we create a voucher of  1000  with we received 100 of discount, so my voucher is only of 900.

now we apply a withholding of 00001  10% to the 900 and we got  800.

but when we apply the second withholding of 00002 2% to the 900, it calculate a withholding of 20 and it must be 18. (it include the discount in the withholding calculation)

by the way how can we chat or make direct conversation here(Becouse some time we have problem and need some one quickly)

Ambiorix