Line used on service contract

Hi
A service contract has a line of use.
The line has an associated counter use.
To counter readings are entered monthly.
Billing claims the diference of reading a meter reading against the previous month.
So does the line use, charge the customer to use the equipment according to the difference found in the two meter readings.
My question is:
As reported or bad pages that should not be billed to the customer.
Of total pages consumed or used should be deducted the number of pages reported as faulty or bad.
As this?

Thank you

Diego