Don't try the Norton Stone - you're far better finding a *flat* (i.e. not kinda flat but truly flat) surface and some wet/dry sandpaper to lap these guys. I found a marble tile from Home Depot or equivalent did the trick. The diamond plates are perfect for lapping, but not necessary.
But however you do it, lapping these are essential. That's true of all hones, but from what I've heard especially true for the PHIGs since quality control varies somewhat.
+1 to the advice re the gent in Poland who sells them if he still is. That's where I bought mine and I was very happy with it. It is also true that these stones are a bit of a crap shoot. Some - like mine - turn out to be excellent finishers while others just simply aren't.
Also, IMHO it would not be worth the savings to buy a portion of one of these stones. They are so (relatively) inexpensive that it makes sense to me to hold off until you can get the full size version. It will make your job easier and the process of finishing much more enjoyable I support Whipped Dog and think Larry's a top guy, but the 1/4 hone just doesn't make sense to me when the full size ones are within reasonable reach. Why pay $15 for a 2x4 stone when you can get the full 2x8 one for $25? Just my opinion.