Which is all to explain why I swatched for this project, even though it will be weeks, and possibly months before I cast on. At least now I can actually do the maths and work it out, instead of just tossing and turning and playing with random meaningless numbers.
The swatch above is knit on 4mm needles, the recommended size, and came out at 21 stitches and 26 rows per 10cm. I went down a needle size, because I thought it looked a bit loose, even though that's still tighter than the gauge on the label. I got 22 stitches and 28 rows. This is a stitch gauge that will do for the pattern, although the row gauge was way out. Following the Denim's instructions I threw the swatches in a hot wash, and the row gauge shrunk, but they also came out a whole lot stiffer, rather than softer. Here's a picture of the washed swatches.
I think I'll risk it. Since I'm between sizes, I'll knit the size that is one inch smaller than my bust, and I'll just have to hope that it all works out in the wash.