When I recently lapped my Norton I had to lower the grit to 180 wet/dry as 2 corners on both the 4K & 8K were slightly lower, & I tried lapping it with 600 grit (all I had at the time) but it was seriously slow work, I ended up putting it down & coming back to it with more agressive sandpaper & then moving to higher grit, finished the 4K with 400 grit & the 8K with 1,000 grit. Once it was really really close I just rounded the edges & called it done. Only tried the 8K side for a touch up & that worked out really well for me.