Chargeback reporting calculation

We are an ISP that is selling backups as a service. It was decided to charge customers based on the Data retained chargeback model. I was wodering if anyone else has experience in chargeback calculations and wether this was the prefered method of charging. Our backup config is Daily, Weekly, Monthly and Yearly backups, and our retention schedule is Daily retained for 1 Week, Weekly retained for 1 Month, Monthlyretained for 1 Year, Yearly's retained for 7 Years.

The problem that we are faced with is that the amount charged increases by one full backup every month for one year, and then it increases by one full backup every year after the first year. Obviously our clients are asking questions and we are thinking of reviewing our chargeback strategy.

It is also difficult to quote the clients based on source data size with this model.

If anyone has experience with chargeback, please advise as to what model you have chosen and what your findings ere with the reporting. How do you quote clients and is there any calculators available for DPA Chargeback.

Any help will be much appreciated.