One way to get super accurate estimates is to ensure that what you do is repeatable. For example get a developer to create the same web page time and time again

One way to get super accurate estimates is to ensure that what you do is repeatable. For example get a developer to create the same web page time and time again with minor variations. And this will be very easy to estimate. Therefore don't automate things and don't make things more efficient. That will help with estimation.