Copying The Last Value In A Range

I have a spreadsheet set up that keeps a running total of the money in my checking account. I've set up the "balance" column to only display the balance if the "debit" or "credit" columns have data. For example:

credit debit balance
3.00 12.00
2.00 10.00
5.00 15.00


______________
15.00

This way I have space to add more entries, without having to copy the formula in the "balance" column every time I add an entry (or have my balance copied all the way down the column when there are no entries that alter it). However, the problem I'm encountering is displaying the balance at the very bottom (underneath the line). I would imagine I need to somehow look up the last value in a range in the "balance" column, but am unsure of how to proceed.