If the result of a appears in cell B5 and that of b appears in cell B6, the value of MU is given by
= (NORMINV(0.99625,0,1) + SQRT((NORMINV(0.99625,0,1))^2 – B5*B6)) / B5........... 
Values for some sample sizes with C = 99% and D = 99.625% appear in Table 1. These values of C and D were selected so that
the one-sided tolerance interval limits would be (x mean – 3 * s) and (x mean + 3 * s) when the sample size is around 250.
Two-sided multipliers (MUL) are calculated by Howe's method with C = 99% and D = 99.25% (in equation 1).
One-sided multipliers (MU / ML) are calculated by Natrella's method with C = 99% and D = 99.625% (in equations 3, 4, and 5).