how to calculate the step deviation

Your question is incomplete. However, as you have asked the question under the Chapter of "Correlation" it seems that you are asking about the step-deviation method Karl Pearson's coefficient of correlation. In this method, the value of the deviations (as taken from the assumed mean) are further reduced by dividing them by a common factor. The following is the formula for the calculation of Karl Pearson's correlation coefficient using the step-deviation method.