If there was a single ideal setting for all Neff fridges, they'd be made with the dial glued into that position (or, more practically, with no dial at all). The required setting depends upon the ambient temperature of the room that the fridge is in, and also on the amount of food in the fridge. (A fridge works more efficiently when it's full).
Buy a fridge thermometer. They're available from all kitchenware shops, or for just £2.45 online:
http://www.cookability.biz/kitchen-craft-fridge-thermometer/b_666.htm?SRC=GB1
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Then set the dial to the position which produces a temperature just below 4C.
Chris