subtract the value by setting up automatically
 

I have a work sheet, it set up by clients @ the 1st column, the 2nd column
each month serivce was charge. it go by 13 periods of a year instead of 12
periods. each month service is based on how long the contract client sign is
between 1 year to 3 years . At the last column is the total value the client
pay. My goal is to set a automatically date or formula on using the total
value subtract the service charges by each month.