in reply to My project estimates are based on:

...when I am told that I must be done.

Then I manage expectations accordingly.

In my company we get a project objective & a deadline that we back into almost every time. Therefore with a fixed amount of time, I scale back the functionality to a level that I can actually deliver in the given time. If it successful and popular then we schedule another swipe at it to add more functionality.

Wait! This isn't a Parachute, this is a Backpack!