I think they know the target they are shooting for at this point. Every single one of these articles talks about training for peak performance, as its described, how to be at your peak on Game Day, with max prep time but also full recovered from training.

Now how they arrive at those values is a great question. The most commonly mentioned system was developed in Australia for Aussie Football.

My guess is that they measure, or attempt to measure movement in an actual game. The NFL didn't allow devices to measure until very recently, so that was either done in camp, scrimmages or preseason games or they went back to tapes and measured by hand and eye.

McCarthy's comments made it seem like after a couple of years measuring, they were implementing changes to the schedule this year based on that data. He has reordered the workouts from Friday to Game Day. Friday is now rest and the walk-through happens on Saturday.