They constructed ladders to reach to the top of the enemy's wall, and they did this by calculating the height of the wall from the number of layers of bricks at a point which was facing in their direction and had not been plastered. The layers were counted by a lot of people at the same time, and though some were likely to get the figure wrong the majorly would got it right .... Thus, guessing what the thickness of a single brick was, they calculated how long their ladder would have to be. - Thucydides, The Peloponnesian War
So when you hear, we don't need to estimate to know when we're done, that willfully ignores all the naturally occurring variances and the event based variances in all our project work. The Greeks knew this, we need to heed their advice.