Proration of meter readings

Dear All,

In our billing module under IS-Utilities, meter readers enter the reading

dates, and meter readings.

Example:

current input

meter reading date = 10.05.2009

meter reading = 2150 kwh

previous input

meter reading date = 10.04.2009

meter reading = 2000 kwh

bill amount = (2150 - 2000) * rate/kwh

We have a requirement whereby customers should not be billed for a period

greater than 30 days.

Even though the meter reader can read after 30 days, we need the bill

amount to be prorated for 30 days only.

Can you please advise how we can do this in SAP?

Thanks and Regards,

Malini