http://www.nature.com/nature/journal/v507/n7492/full/nature12961.html
Fascinating. I recall not too long ago the general consensus was that modern humans had no Neandertal DNA, now they say as much as 40% got transferred.
The difference is, in the past 10+ years, Neanderthal DNA has been extracted, analyzed & compared to our own.
The major findings were:
Bottom line: much of the numbers you arrive at depends on what exactly, and how, you measure, but the overall average of 2% Neanderthal DNA in non-African humans seems to me a fair estimate.
Your 40% refers to the fact that not everybody is the same 2% Neanderthal.
So, some of us, especially (but not only) us males, may be more Neanderthalic than others. ;-)
Some typical Neanderthals, ancient & modern:
Oooooooops!