Modify the control limits for Acceptance Individuals Chart

Stat > Control Charts > Acceptance Charts > Individuals > Acceptance I Chart Options > Limits

Display additional σ limits at

In These multiples of the standard deviation, enter one or more values to display additional standard deviation lines on your control chart.

For example, you can display additional limits at ±1 and ±2 standard deviations. Values that you enter affect only the line position on the control chart, not the test results.
Default 3 sigma limits
Additional sigma limits at 1 and 2 standard deviations