The Lily of the Valley Scarf is finished and ready to block, so I’m free to start the first of my selfish projects. 
It’s a long time since I knitted anything that needed to fit to such a precise tension. But as I was not going to cast on the entire amount of stitches for my size and then knit for a few inches and discover it was too small, it had to be done. I started off with needles in the size stated in the pattern and knit a pattern repeat over fifty or so stitches.
This proved to be too small, so I tried 4mm needles, which was also too small so I had a fit, calmed down, washed the swatch, measured it, had a fit again and then stopped poking at it and left it to dry. Once it was dry, the tension was perfect and I stopped scaring everyone with my swatch induced rantings.
Despite the tension traumas, I am definitely loving the New Lanark yarn so far. It’s got a lovely stickiness to it, which reassures me that it will steek nicely. It also softened up after soaking and while it’s not going to be like wearing cashmere, I am never going to wear the vest against my skin, so I don’t have to worry about the chance of it irritating me.
So I’m all set to start, except that:
a) I don’t have a long enough circular needle in the right size, and
b) My printer is out of ink so I can’t print out the charts I need for my size. I printed the pattern out when I first bought it (probably a couple of years ago now!), but only the directions for the larger size range. I’m knitting the 42″ size, which is the biggest of the smaller size range. At least if a is resolved before b, I can cast on and work the ribbing.
And in the meantime, there’s blocking to do! Plus, it’s not as if I don’t have plenty of other projects to finish while I wait… 
Are you swatching in the round, or carrying loose strands around the back? One of the reasons I haven’t done much colourwork yet is that swatching is a pain in the ass for me.
If you’re carrying loose strands around the back, how in heck do you keep the first and last few stitches from being totally loose? It drives me crazy, and I’m convinced it’s messing with my tension because I’m not knitting as I would be in the actual project.
Glad to see that you worked it out in the end. It’s not often that I swatch, mainly because like you I get very frustrated when something doesn’t go to plan. I know it does make the finished piece that much better, but it’s such a pain. Good luck!
i don’t mind swatching, but i do hate doing it properly and waiting for it to dry when all i want to do is cast on right now right now. the swatches look good!