How to add Custom discount section in sales invoice section

Can someone guide me on how to add a Discount percentage field in the sales invoice next to each entered item?
In my business, the discounts on a particular item is not structure/fixed and it varies from customer to customer. Therefore I am looking for a way to put in discount percentage according to my convenience at the time of raising a sales invoice as pre-defining discount rules do not work for me.

You may try to open Customize Form, Sales Invoice Item, select “In List View” for “discount_percentage” field.
PS: You may need to remove some other fields as well. We can only show 11 columns.