This is an example of what I'm talking about.
What's more of a waste of your training? To be work in a career field only 60% overlapping with your training, or to be unemployed (with 0% overlap) ?
If you want to read more on this look up "search costs"
http://onlinelibrary.wiley.com/doi/10.1111/j.1465-7295.1976....
Would be interesting to count one as being only 60% employed, in the example you give. So if you had 100 million people all employed at 60% fit, that's an 'effective' employment of 60 million. Of course, I have no idea how one might measure that.
But I am not an economist, there may be similar (or superior) measures for underemployment.