Credit Management not consider Tax Value

Dear Gurus,

My credit management is working fine, but system only cross check the net value against credit limit, not accumulating the tax value in it.

lets say my credit limit is zero 0 dollar, order net value is 1000 $ and tax is 100 $, but while credit check system gives the message that "credit limit has been exceed by 1000 $" i want it should be give message of $1100.

Please guide.