Value of whisker in boxplot for 99.7 coverage
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I am trying to identify outliers from a boxplot using MATLAB. The function has a default whisker value of 1.5 that provides +- 2.7*sigma or 99.3 coverage. 
However, I want 99.7 or 3*sigma coverage. What could be the value of whisker in this case?
I did not want to make a random guess, so need help from you guys.
Thanks