Hi,
Have a look at:
http://imagejdocu.tudor.lu/doku.php?id=plugin:color:chart_white_balance:startBe aware that the red color on the colorchart will not have a R value of 255. It will be approximately 175. So it is close to your measured value. See:
http://www.babelcolor.com/download/ColorChecker_RGB_and_spectra.xlsRegards,
Peter