EBIZ TAX CALCULATION ON Assessable Value in sales order

Hi,

We have following scenario where tax needs to be calculated on Assessable Value on sales order.

Order Line Amount :-100

Discount Amount:- 20

Order net Amount:- 80

Taxable amount:- 90(Assessable Value)

Tax Percentage:- 5% on (Taxable amount :- 100 )

Tax Amount:- 4.5

Following Invoice needs to be created through Auto invoice process:-

Invoice Amount:-100

Discount:- 20

Net Invoice Amount 80

Assessable Value:- 90

Tax Percentage:- 5% on (Taxable amount :- 100 )

Tax Amount:- 4.5

Assessable Value needs to be define on sales order.

Please let us know how can we achieve above scenario

Thanks