Hi,
You said that you were able to create 100% in new column, right? Let's call your new column as KF_NEW_COLUMN..
Did you already try this formula?
(NET_INT_ACTUAL = 0 AND NET_INT_BUDGET = 0) * KF_NEW_COLUMN + ((NET_INT_ACTUAL <> 0) * NET_INT_ACTUAL)
Regards,
Loed