Personally I have never touched a new XRAY diff - they have never let me down in 4 seasons - maybe the one's sold in England are better than those in Holland?
Tighten it up so it is "loose" but you can feel resistance.
On the bench - as per ZZOEBL.
I then tighten 1/8 turn and repeat
I keep on repeating until I get to the tightness I want.
Alternativley run the car on the track, not too fast, for a couple of laps, tighten, repeat.
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Then run the car for 5 mins at normal speed and recheck tightness.