Tax calculation on Purchase Order based on assessable value instead of line amount

We have a requirement where the tax on PO should be calculated on assessable value instead of line amount.

 

S.No	Product	Unit	Selling Price		Discount Price
			Basic	GST Tax	Basic	Total
a	b	c	d	e=d*0.17	f	h=e*f
1	Item A	Liters	100	17	90	107

We have a senario where the Item retail price is 100/- and 17% tax. The discounted price of this item is 90/-. We want to calculate the tax on retail price intead of discounted price. Above illustration FYR. How can we do this ....?

Regards,

Haseeb