calculate running balance?

Content

I'm trying to get a report to compile a running balance.
I have custom fields to capture all the data, and I know I need a custom script for the calculation, but I'm not sure how to write it.
My columns are:
Month; Total hours; Hours Used, Balance Remaining

The Balance Remaining is where I am stuck.  Below is an example of what I want to build.
Balance remaining = Previous Balance Remaining - Hours used
where each new row balance is calculated on the previous balance.

Month	Total Hours	Hours Used	Balance Remaining
Jan	500	40	460
Feb	500	20	440
Mar	500	60	380
April	500	75	305

Any help with the script is much appreciated.