PowerFC knock reading using standard Knock sensors on a Skyline should be set with a warning threshold that when hit will light the management light (depending of course on the mapper)

This limit is usually set to 40 on most cars, but I think it's 60 on Skylines. Anything up to this amount is 'OK'. Spikes beyond this amount may be acceptable too if they really are just spikes and you don't actually 'see' them occur.