When should you underestimate?

Answer 1

If the consequences when the estimation or calculation error is at its minimum are very seriously negative. For example, suppose the lifting capacity of a crane is 10 tons to the nearest ton. That statement implies that the capacity is anything between 9.5 and 10.5 tons. You must use the underestimate: 9.5 tons since that could be the true limit, and the crane could fail at anything higher.