I have two questions:

1) I have three fields, "Opening Balance", "Amount" and "Closing Balance".  I would like to have a formula in the "Closing Balance" field calculated based on another field (Payment or Receipt).  If Payment, it would be "Opening Balance" - "Amount" and if Receipt, then it would be "Opening Balance" + "Amount".  How do I do this?

2) I would also like to have "Opening Balance" be auto-filled in a new record, taken from the "Closing Balance" of the last record.  How do I do  this?