Create a summary field. Options to select for summay field. Total of, amount and running total.
Since you have a field for debits and a field for credits, (I will assume a debit should increase the balance)...
Define a calculaton field, cBal as:
Debit - Credit
Then define a summary field, sBalance as the total of cBal. You can select the running total option to get the typical check register type running total to display in your ledger.