Discount as Absolute Value

On Sales Order Entry Form the Line Discount is in Percentage. I want to add another text box that should take input the Line Discount as Absolute Value.

Upon Lost Focus of that Text Box the Discount Percentage is automatically calculated and updated in the Discount Percentage Box through Javascript.

Can any one help me in that regard?

1. From where can I add another text box?
2. From where can I configure the Javascript code to update Percentage Box through Absolute Value Discount.

Regards.