How is fare divided over legs?

If Delta cancelled the first (domestic) leg of an international trip, how is the refundable amount for that leg determined?

We flew to Europe with refundable tickets, with the first leg from BHM to ATL. Delta cancelled that flight. The receipt does not itemize an amount for that portion of the trip. How much will they refund? And because we had to drive to ATL to connect, we had to cancel the inbound leg (ATL-BHM).