The specification nominal is NOT included in the calculations for *statistical** process control charts. If you are using an Xbar, S or Xbar, R chart the center line is the grand average. If your subgroup size is always the same then you can simply average the subgroup averages. If your subgroup size varies then you have to calculate the average using all of the individual points and not just the subgroup averages.

*There are some control charts - not statistical - that utilize specifications and I'm sure Bob will point you to one for tool wear...