Zones and
WECO, (Western Electric Company Rules)

To create the zones, first find the upper and lower control limits and then divide the section of the chart between the UCL, (upper control limit), and the mean and the LCL, (lower control limit), and the mean evenly. You do not calculate the zones individually.

The established way to trend these charts is by using the WECO, (Western Electric Company), rules

^*You ARE OUT OF CONTROL IF . . .

• A single point falls outside the 3 Sigma limit, i.e. beyond Zone A
  
• Two out of three successive points fall in Zone A or beyond, the odd point may be anywhere
  
• Four out of Five successive points fall in Zone B or beyond, the odd point may be anywhere
  
• Eight successive points fall in Zone C or beyond
  

For additional information see What are the WECO rules for signaling "Out of Control"?

Another way to envision the zones are shown below.

There are many different rules that one could use. These type of rules are to evaluate trends. Since every process can be different, the rules are sometimes modified as to not stop production for an out of control condition when the design margin is still being adhered to.

Pre-Control Limits

Pre-Control limits are sometimes used instead of the calculated upper and lower control limits for control charts. This is done due to the fact that some processes may typically run skewed, or shifted from the mean or median. There are established formulas to calculate pre-control, but as with all cases, to include the WECO rules, you may have to modify them to fit you particular process.

Using ZeroRejects, you can easily see where your data is in relation to your  specification and design margin.

For addition information...

Control Charts as a Tool in Data Quality Improvement (U.S. Department of Transportation)