Thanks guys - my enlarger has an adjustable negative stage; the lens board is attached to that. Difficult to adjust and then tighten without it falling back off. I got things basically aligned with a spirit level (bubble level in the states!) and found I needed to shim the lens board. Using an El Nikkor 50mm 2.8. After using the Magna, I'm amazed at the magnifying power of the micro though.
I'm aware of the peak focuser, but will try to get by without one for now - they seem to get grabbed up quickly when they appear. I see a slight focus shift when I get about 4" from center (on a 16x20 so I'm fairly close to the edge). You can still tell that it's grain and see the structure - it's pretty minor. I'm sort of mentally designing something like a lensboard with tapped holes and small thumbscrews to fine tune the lens board angle - shimming's such a hit & miss process vs. "dialing it in".
I feel like I have the thing reasonably close and to a point that all this may be moot when stopped down, and I'll do some materials tests at 20x24 and see how everything goes. At some point I'll need to build a sort of level-adjustable combination easel & dev. tray of "rather large size" (I'd like to use materials up to 5'), and I'm curious to see what effect DOF has going very big. Anecdotally I've heard the focus tolerance increases for projected prints, so I may be fine. Thanks again!