This is where the yarn pooped out on the Brambles beret:
That's with 15 rows and 11 stitches left to go. I calculated it - I need enough yarn for 1,262 more stitches.
Now, I could of course go out and get another skein to achieve this. But this is supposed to be a stashbuster, not a stash enhancer, and if I get a whole new skein, I'll end up with more yarn than I had to begin with. Another possibility is to scrap the idea of using the Cascade, and instead turn to a full skein of red Briggs & Little Regal that is also in the stash. However, the downside to that idea is that the Regal is kind of scratchy, not at all like the yummy smoothness and squishiness that you get with the Cascade.
So I whipped out my pencil. I calculated that if I ripped out a bunch of it and followed the instructions for the short length instead, I would save myself 1,064 stitches. Good, but not enough. I could then reduce the height of the band by two rows, and that (accounting for the fact that the band is done on smaller needles) would get me another 182 stitches, for a total of 1,246. That leaves me 16 stitches short of my goal, which I can probably achieve just by making my stitches a little more snug throughout.
On the other hand, I actually do want to have the medium-length version, not the short-length version. So my ideal solution is really to find someone with a little bit more Cascade 220, colour 9422, (hopefully) dyelot 708, in their stash.
I guess it's off to search Ravelry. I wonder if I can type and cross my fingers at the same time...