I personally like 16-18 seconds icing best for flooding and can go pretty close to the border with that, depending also on cookie size and shape. I use either icing with meringue powder or with egg white powder - and don't really notice a difference between the two. No further additives, though.
Outlining works best with medium icing a bit on the stiff side (does that make sense?), but I avoid outlining whenever possible, as I don't like the look of it. It always shows too much.
But I guess this method won't bring you close enough to the edge for your purpose...
How about making an iced border after airbrushing? Or maybe a fondant snake which you can use as a temporarily boarder and remove after?