This is pretty common knowledge to most players here, but since you made a whole new thread about it, we might as well expand upon the topic with some details. First off, diagonals count for more distance than horizontal and vertical intervals. I also think that, in case of completely even distances, some directions are favored over others, but I'm not completely sure of this. Maybe somebody else will chime in with more info.
Next up, feasibility. Having unsupported settlers is great, but this trick might not be the most profitable thing to do if your settlers are absent from work for too long. So you should generally only make use of it when your settlers have no more (currently needed) improvements to make and no good land to settle, or if you have enough cash to rush build the settlers back to life quickly. But if you can afford that, you probably don't really need unsupported settlers, but rather you want them just so you can get some extra population points.
And, finally, safety. Besides possibly slowing down your development if not used at the right time, this trick is also a liability--you can lose the settler or, even worse, let the AI steal important tech. So you should always bring defenders and use terrain to your advantage. Since you're not really looking to grow them, you might as well build these cities (or should we call them slave camps?) on hills, mountain tops, etc.
Personally, I get my NONE settlers by disbanding badly placed or indefensible AI cities that I conquer or flip to my side. I also usually don't build too many settlers of my own, which means that the ones I have always have a lot of work to do, so I rarely use this trick, but I can see it happening in the lull just before railroad. Or maybe I can do a mass conversion of settlers from owned to NONE on that tiny faraway island, just before I found my real colony there (I like settling small islands and I usually do it later in the game, when I've developed my mainland and I can ship a whole bunch of settlers to build the rails and farms quickly).
There is a downside to NONE settlers though. It hurts so much more when you lose them to a random AI sneak attack.