Well, I seem to have spread some confusion, so I shall attempt to clarify.
I don't see what the particular problem is here. 1 tile should be all that's needed.
1 tile gap is all that is needed for the plug to drop. That's certainly true. However the dwarves needed some way of getting down there. In retrospect I could have tried to trap one of the mining dwarves down there by digging two levels out and not providing a way out. I didn't think of that until later, though. That's a sure fire 1 dwarf kill though.
Why are those stairs there?
So the dwarves could get down and out (see above).
The method given in the wiki would have a notch taken out of the side of the plug next to the stairs. Or you could have the stairs going down outside of the gap. Either way is esthetically displeasing to me.
You are unlikely to be able to make a decent plug without breaking the surface unless you are working under a hill.
Yeah, I meant the surface past the outside of the gap. For example if you have stairs going down and a tunnel across. I get what you mean by the hill thing, though. You could have a natural unbroken surface if you have enough z-levels before the aquifer to have a 't-shaped' plug. One with a higher radius at the top than the bottom. That sounds quite cool, actually, and my current fortress may be able to do that.
I think what I may do is have a cross-shaped hole in the center of the plug with central stairwell and floor hatch to reach it. That would keep the symmetry better. It would leave 5 tiles of 7/7 water to deal with, but they could easily be pumped out as they would not be adjacent to any aquifer tiles.
