Turns out my maths wasn’t quite exact enough.
All the way along the cast-off row (which is a stretchy one and takes even more yarn than usual) I was thinking ‘but actually, there’s probably enough, no it’ll be fine, the law of satisfying knitting results dictates that I should be able to blog a picture of a finished shawl and a few inches of leftover yarn, so I can be all “look! immaculate knitting maths!”‘
But no. However, I had a good foot or so spare yarn from my cast-on! I could weave it in and snip it off and then add it in to make up those last few stitches! Amazing!
Bah. It’s all fine, I’ve used a tiny bit of leftover Socks Yeah! grey from the Twyllas and the shawl is blocking as I type. It just isn’t that perfect rounded smug finish I was hoping for. Which of course serves me right.