VatAmount value

In purchase posting, I must change VatAmount value, because the VatAmount must be calculated in a different value than lines total Amount. where can I implement this new VatAmount? In wich table, CU,....???

In  Purchase Invoice Report, the VATAmountLine."VAT Amount" = Amount-("Purch. Inv. Line".Quantity*x) where x is a Constant = 0,4.

Where can I implement this formula to  have the correct VatAmount in all financial tables?

thanks