Yeah, the last factor I didn't quite follow. Seems like the original author felt like 8000 was too large, so he needed a bullshit excuse to scale it down one more time.
I believe what he meant was the stuff in between 'starting' and 'finishing', i.e. doing the work. Lots of people start and do some of the work and never finish. Many also start, do a huge amount of work, but also never finish. It's 3 distinct phases, each of which represents a progressively smaller proportion of people.
What does this prep entail? What more needs to be done apart from having the motivation,starting and finishing building the damn thing?