I think the commonly claimed rules about adding increment your unit of time or multiplying by x are kind of childish and a wasteful. Estimating time to complete

I think the commonly claimed rules about adding increment your unit of time or multiplying by x are kind of childish and a wasteful. Estimating time to complete is difficult because individual items have high variance. If you have 3 items that are likely to take 2 hours +/- 2 hours, estimate 10 hours for the lot. Variance of the total goes down by a factor of n^(-1/2). Explain why you do this. Flag when variance is screwing you anyway.