Since 2007 the production has changed locations on the Nortons.. this is why you hear so many different opinions
Back in the past the 4k and the 8k were made in the USA and the lapping went rather fast
Then the 4k switched to Mexico and we heard of people lapping over 1/8" off to get a smooth surface
The 4k switched to Italy next and things got slightly better
BTW there is no absolute with any of this, you lap until you get a smooth even surface, but these are the trends we saw
This is also different for the single grit Norton vs the 4/8 :p