How much sugar and corn starch is in commercial icing sugar? If anyone knows about the percentages of sugar &amp; corn starch to make icing sugar for commercial purposes, please let me know. How important is it for these percentages to be accurate?
Powdered sugar (as it is known in the US) is approximately 3% cornstarch by weight.

See, for example, United Sugar's website.  This is typical of all manufactuers, however.