If you provide only one specification, and you want to include 99.73% of your measurements, you do not need to change the 6 to a 3. Minitab divides the constant by 2 for these calculations.
Minitab can calculate the probabilities of misclassifying product when you enter at least one of the individual specification limits.
Enter a known value for the total variation, which is the part-to-part variation plus the measurement system variation. Use the drop down to specify whether to use the historical standard deviation to estimate the process variation or to use the parts in the study to estimate the process variation.
When you select Use parts in the study to estimate process variation, Minitab displays %Process in the output. The %Process column shows the percentage of process standard deviation taken up by each variance component.