I got meanwhile an answer from Steven Kapsos from the Economist, Employment Trends Unit at ILO about the calculations.
He pointed out that there are details in an article by him at:
http://www.ilo.org/public/libdoc/ilo/2005/105B09_255_engl.pdf

I didn't understand a couple of things in that article and wrote him an email about that. I will let you know more when I get his reply.