Your layout shows a series of repeating fields with 24 repetitions. To keep with that design approach, (I strongly recommend you don't), you'd need your discount field to also be a repeating field with 24 repetitions. You'd then need to edit your existing calculation fields to include the new discount field in their expressions.
I strongly recommend you restructure your database to use a portal to a related table of records for your line item entries on your invoice. The invoice template that comes with filemaker 11 is a decent example of how this is done. (If you don't have filemaker 11, you can download the 30 day trial and get the templates that way.) The resulting system is many times simpler to implement and much more flexible.