My calculations didn't work out quite as planned. I knit 44 rows of raglan but I ended up with a piece about 2cm short. It is entirely my own fault for not measuring the row gauge more carefully. I ripped back and worked out that I needed to knit 50 rows to get to the right length. This is including 20 rows of decreasing 1st at each end. If I'd wanted to be really, perfectly accurate, I would have calculated how to do this every 2.5 rows, to be sure of producing a perfectly smooth slope on the raglan. However, I was still taking short-cuts so I took the simpler strategy of decreasing on every knit row *except* for on three of those knit rows, which I distributed evenly throughout all the knit rows available. In other words, on every fifth knit row, or thereabouts, I just didn't decrease. This means there are 'blocks' of about 3 rows where no decreasing is happening which doesn't make for such a perfectly smooth line on the raglan edge, but I think it can be teased into position when I stitch on the straps and block it.
So there we have it. A sweater back with an acceptable, if not perfect. raglan edge. The back is exactly the required size: 52cm high, of which 16cm are raglan, and 20cm wide at the top on 37sts.
While I was ripping back and reknitting the raglan on this thing, I tried out a few methods of decreasing. Here's the formula I finally decided on. Left-leaning decrease: k1, ssk. Right-leaning decrease: k2tog, k1. This seems to be the method that produces the most similar appearance of the outer-edge stitches on each side of the work.