Hi Santosh,
i would like to summarize your question, in order to have same understanding on your request.
You want to have a new tax code (6%) and calculate tax amount based on: total amount before tax - discount + packaging & forwarding amount. Please confirm it.
If this is your request, actually you don't have to create a new one but using existing one. Standard SAP Business One already calculate tax amount according to above calculation.
It would be helpful if you can share with us how the system calculate your tax amount currently, because personally i'm not working within the area which is using excise tax.
Note: if you want to create a new tax code, go to: Administration -> Setup -> Financial -> Tax -> Tax Group.
Regards,
Lailus