I think this is an excellent idea. Calculators were just being brought into the school curriculum when I was 8 years old (to some controversy at the time!) and we were told always to use the the three E's: estimate, evaluate, evaluate. The idea being that first you guess roughly what the answer should be and then after calculating it, you check it again to make sure you havent made a keying mistake. I've always thought this a lesson well taught that I've applied to many automation tasks since.