I don't know the NSR specs but my vintage Vespa's have 2.0mm service limit on the ring gap, I found them at 3 mm once and the bike still ran fine: 45mph on a 50 y.o. bike. Put it back together and it still runs like a champ!

I say tun it, 0.495, 0.500 or 0.508 is virtually the same gap, I doubt snap-on gauge can provide such an accuracy. if you had a 0.490 feeler it would "feel" the same as 0.508 going through the gap,