Do not know what you mean by xcmc.

For determination of cmc by conductivity measurements see e.g.

http://151.fosu.edu.cn/hxsy/wulihuaxueshiyan/my%20web/biaomianhuoxingji%20y.htm

in practice (with systems which containing some 'impurities') http://151.fosu.edu.cn/hxsy/wulihuaxueshiyan/my%20web/biaomianhuoxingji%20y.htm you will see that cmc usually does not mean a defined point but a concentration range starting at formation of first micelles enriched with the most surface active species up to the concentration where micelles composition is the same as for the components in solution.

thank you Mr. Titus Sobisch