Do not ask for honest estimates and then punish the person who gives you one.

An estimate is a forecast, and forecasts get shaped by what happens to the forecaster. If the realistic number is met with disappointment, pressure, or a negotiation, people learn to give whichever number avoids that conversation. You then get optimistic figures, missed dates, and no way to plan, because the honesty you asked for was trained out by your own reaction.

Watch your face and your first sentence when a number is bigger than you hoped. Ask what drives it rather than whether it can shrink. Make giving a true estimate safe.