I am watching Glen's video about lapping a brand new Norton 4/8 and I am confused..

In his video he uses a DMT 100 to do the initial lap then finishes on the 325, I thought the idea was to get it flat and smooth and you could do that with a 325 only, My norton's are brand new and I have a DMT 325 on order..

So can I use the 325 only or do I need to get some weird sandpaper to do this (or buy the expensive DMT 100)

*confused