Root Sum Square(RSS)

hello,

     everyone,

     about statistical method(RSS) applied in tolerance analysis,can anybody tell me its connections with CPK level.Assumed that B=A1+A2, and it is already known that CPK of Dim A1 is 1.33, Dim A2 is 1.5, now I wonder what the status of the CPK of Dim. B would be, do we have to evaluate it through maths method or is it defined in any other way  or  is it just unknown to us? I need to compute standard deviation with CPK,but there's no more information can tell me how to,since I don't know what's the CPK value of Dim B is. Help me,please.Thanks!