how to apply GST based on after discount price in Purchase Order

Dear All,

I Have a requirement.

Example

There is One PO and Have One Item.

Item Price = 100 and Quantity = 1

Then Gross Price will be 100 and then apply Discount 10

after Discount Price will be 90.

So, How Can I apply GST conditions based on after Discount Price like if After Discounted Price > 90 then apply 5% GST elseif After Discounted Price < 90 then apply 12% GST.