you cant directly reference the table fields from the label design. Also the options for the field drop down is hard coded in the program business rules and not stored in a table.
You could prepopulate the detail1 field with the saving percentage though
something like this might work:
update item set item.SubDescription1 = ABS ((item.MSRP - item.price) / item.MSRP)
You might have to change it to suite your needs. Dont know if your are using the MSRP in the item properties. Also dont know if you want this for every item. I would suggest a where clause at the end of the update statement.
for reference. detail1 = dbo.[item],SubDescription1
detail2 = dbo.[item].SubDescription2
etc.