A Ninja Maths Master should be able to make quick estimates:
Q1: How many people can fit onto a double decker bus?
A1: Between 65 & 75 is a good answer
Q2: How many grains of uncooked rice in a cup?
A2: Between 5,000 and 10,000 is a good answer
If a child gets an estimate way off target they should explain how they arrived at their estimation. A guess is insufficient, it must be an estimate arrived at using thought and sometimes a pen & paper.