You're going to find that you have to remove a lot of surface area to get past the graininess of the Norton (or at least I did)...You'll know once you hit the good stuff!!

also, you'll want to draw a grid w/pencil on the surface of the hone prior to lapping, so you can tell if there are any high/low spots. I personally had to do this 4-5x on my Norton before I was able to remove all of the grid at once. It's a bit of a process, but it's worth it!!