DEVELOPMENT SERVER
Rules to Happy Clients

Do *not* wait until you have started to exceed your estimate, before you notify the client that the release is running late.
 
Let them decide and avoid conflict by sending an email like this:

To: Mr Northwind
Cc: David (Project Manager)

Dear Bill Northwind,

As per our conversation, Release 3 of Target Contacts will take longer than expected. The data migration of the Coverage Data is more complicated than originally anticipated because an external database (Media Disk) also reads from the coverage table. I am revising the estimate accordingly to 16 hours.

Regards
David

Figure: A sample of an email that informs the client that the estimate will be exceeded
As soon as you realise that any of your estimates are likely to be exceeded by a margin of 10% or greater, then let the customer know ASAP by phone and by email (using the 'as per our conversation' rule). This will ensure that the client is fully aware of any problems and has a chance to decide an alternative action. 

Never keep the client in the dark when you exceed your estimates, it will only arouse suspicion and mistrust when they see the project deadline woosh past.

SSW eXtreme Emails! that automatically generates emails to notify the client of an exceeded estimate