It's impossible to say what the force is, but I would say that if you can turn it, it's still loose. Stoff's method for break in is excellent. Ball diffs work with very high point loads on the balls, and that causes them to make 'tracks' in the diff plates. The depth of those tracks is enough to effectively loosen the diff, and most of that effect occurs during the first run. Following Stoffs method eliminates a lot of that problem, and it is still worth checking again after the first 'proper' race with the car.
Ball diffs last longer, and work better, if they never slip. In off-road, with the slipper clutch, diffs should have no slip and be glass smooth. After a couple of rebuilds, you'll get the hang of how much force to apply, but on first acquaintance, if you can move it, tighten it!! HTH
