About % ¤ of the area is between z = -3.5 and z = 3.5 (or within 3.5 standard deviations of the mean). Basis for the Range Rule of Thumb and the Empirical Rule. In Exercises 45–48, find the indicated area under the curve of the standard normal distribution; then convert it to a percentage and fill in the blank. The results form the basis for the range rule of thumb and the empirical rule introduced in Section 3-2.