The fact that the conventional estimate of spare capacity is based on the unemployment rate reflects an implicit assumption that if you have a job you are pretty much fully employed. In decades past, this might have been a reasonable assumption. But it is not a realistic assumption in today's modern flexible labour market.
As more people work part time, it has become increasingly common to be both employed and to work fewer hours than you want to work. In the 1960s, less than one in ten workers worked part time (Graph 1). Today, one in three of us works part time.