A more correct measure is a much simpler one: the aggregate product of all these companies over the GDP. Note that this aggregate product is of course a component of the GDP, so you can express this neatly as a percentage. I see only one major accounting complication in constructing this metric, and that's the inclusion/exclusion of foreign products and subsidiaries. (See e.g. offshore revenues that are not repatriated, and therefore not part of the domestic product calculations -- I think?)