Thanks for the input guys. Very helpful.
Anyway I made some progress today. In fact I think I got things bang-on and am concerned that it can't be this easy
I measured the hub & rims per the instructions on the link provided by
3alarmer with a steel tape and a straight edge. Not ideal but all I could do without vernier or similar.
These measurements over-rode the 'cigarette packet' ones in various ways. IE the distance between the flanges isn't needed and the diameter of the flanges is not needed. It's the diameter between the spoke hole centres that you need.
Anyway I punched in the data from the built-up wheel to see if it came up with the same length spokes as already fitted and it gave me 273.9mm & 268.6mm for the 274mm & 270mm that I measured on the spokes themselves. Pretty close
crude measurements from donor wheel input in calculator tell me spokes should be this length (3x column). They pretty much are
Bouyed by this apparent success I then changed the rim data to 542mm which I measred my blue rim ERD to be and it tells me I need to hunt for spokes 251mm & 256mm long. I wonder if the calculator accepts Imperial measurements as I see the older spokes are often listed in Imperial length rather than metric
I'll measure again with proper instruments before buying spokes but looks like I need 251mm & 256mm for the blue rim
On the subject of the donor wheel and my assumption that it is correctly spoked. This may not be the case as it looks like it was built long after the hub was made. The AG hub shows 4 76 but the Rigida Superchromix rim appears to be from 11 84. That's not to say it's been laced up incorrectly but it does suggest it's not 'factory'
picture taken from centre of wheel shows Dyno-side flange offset further than drive-side
rim from donor wheel appears to be from November 1984
built around hub from April 1976