Lapping techniques/basic stone questions

When I bought my set of waterstones (King 1k, 4k, 8k), I purchased with it a Norton Flattening Stone. After lapping the 1k and the 4k I did a visual inspection and noticed the flattening stone becoming "out of shape". I'll say the "Northwest" and "Southeast" corners are at the same height as each other, but Northeastern corner is higher than these, and Southwestern corner lower! Accompanied by a concave dip running down the middle(lengthwise)! Needless to say I was bummed - I assumed, based on the reviews I'd read of the 4k/8k combo as a beginners hone set up, that the flattening stone would also be of good quality. So, this leaves me with a couple of questions.

a) First and foremost, did I make a completely bunk purchase, or is my lapping technique at fault? Is this specific stone not intended for the water stones I have? Has anybody else used it with better results?
b) If it's not just a poor quality stone, is there a guide or method I can read up on to re-flatten it and give it another go, paying much better attention to my technique?

I've read the guide on lapping with wet/dry sandpaper and will be giving that a go (on my waterstones) in the next few days and see what kind of results I can get with that. Living in a small apartment with a room mate, however, I'd like to have a method that takes a bit less time and makes much less of a mess that this appears it's going to make, so I'm contemplating buying a DMT hone to use as a flattening stone. The Hone Lapping 101 recommends the DMT 325, does anybody agree/disagree with this suggestion. Reasons for whichever?

Being the stubborn mule that I am, I did hone my razor on these stones anyways and got some (I feel) decent results for my first time. It was close to, but not on par with, the razor I had bought in the classifieds in terms of shave quality. However, now that the shave quality is starting to diminish I'd like to have another go at the honing process, from bevel set to finish, and just want to make sure the quality of tools I use reflect the quality of the shaves I want.

Sorry for the longwinded, and possibly hard to read post, and thanks in advance for answers or pointing me towards the correct guides!

You cannot go wrong for a DMT 325. Is it the best choice? I don't know. But it will do the job, it will do it well, and will likely last for a long time.