Whenever you estimate meter readings, there is always a risk that your estimates may be significantly lower or higher than the actual amount used. This can happen any time there is an unusual change in the usage for a given period; such as when ownership of a property has changed or when an owner has been away. This is usually resolved within the next period, when you post the actual reading again.
However, if the next actual reading has not recovered from the original estimate, you will need to decide how you wish to resolve the situation. Your resolution may be to bill the minimum for the service until the original estimated reading has been reached or to adjust their next bill as your policies permit.
It may be a good idea to review your current policy or establish a consistent policy in advance to apply in these situations.