in reply to My project estimates are based on:
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!