It’s up to you how you will define your slicer settings.
But in most cases your 3D prints never have single wall on outside, additionally few walls adds-up and make over extrusion more visible.
Personally I’m using single wall when doing some coarse tryouts, or checking for under extrusion, but in final fine tune tests I use more than single wall.
When I’ve run the test previously, with the full model, I can’t see any basis for needing to tune it down particularly fine: there’s a band of a centimetre or so where I really can’t see any difference. I usually pick a spot somewhere in the middle.
I just ran the test with the model sliced in spiral vase mode and it seemed to work very well, at least in terms of what I just said.
I’m open to the suggestion that I must have something else set up wrong, which is why I’m not finding this such a critical parameter, but otherwise, I think this will do for now.