+1 DMT 325. In my limited experience, the DMT 325 has been way easier to work with than sandpaper on glass which was the only other lapping method I tried. I ended up with a 4"x10" but the 3"x8" is super too. I'm glad I got the 4"x10" but that is entirely personal preference. With a Norton, Naniwa or other brand of lapping plate, I've read that you need to lap it right out of the box to ensure it's nice and flat to begin with. Then if I am not mistaken, it needs to be lapped regularly as you use it, although I am unclear at what intervals this needs to be done.
Here is one of the threads where gssixgun gave us his input on this topic:
http://straightrazorpalace.com/honin...ost942686Enjoy! :)
If you do decide to use the screwdriver method consider consistency. When I did mine, it didn't take much as I had only one high spot, I laid the round shank of the screwdriver across the whole plate and ran it along lengthwise about 4 times so it got broken in evenly and not just in one spot. Maybe I'm just too meticulous :P