John,
I had a look at the other thread on this forum where you described the taught wire system on your lathe and also scanned through Borisov's paper, though I must admit to not studying all 170 pages in detail. Cunning use of cheaply available sensors and I can well understand not wanting to involve adjustments to both the wire and the laser. I was thinking more of Joe's desire to check the horizontal flatness of the beam from his commercial laser device when the 2 wire idea came to me.
As someone who plays with a DIY CNC router as just as one of several hobbies I'm always on the lookout for ways of accurately aligning my machine without having to buy several expensive measurement devices that will only ever get used a handful of times. There's also a great deal of satisfaction from devising a method which is inexpensive but effective... which I haven't quite managed yet. I only cut wood, for the present at least, so my current aim is achieve 0.1mm accuracy. So I shan't be worrying about the sag in the middle of a 1200mm taught wire anytime soon

Kit