For a really great diff, I have a few tips.
1. Install as many balls as possible in the spur gear, 12 balls in mine for example.
2. Fit a second belville washer. Confirm that the one touching the bearing is narrow-side against the bearing, and the second is narrow-side towards the nut. It won't work if the wide side faces inwards to the bearing, and the washers must be touching on the wide faces.
The second belville gives you much more range between locked up and fully loose. With only one, there's only a tiny amount of nut adjustment range that is useable, making the diff very sensitive to tune.
I run my diff as loose as possible while not allowing any slip (hold both wheels and try to turn spur gear).
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