Hi,
>>> We acquired some of the calibration kit from a sale of RV's that we purchased for spares.
Ah, I hadn't noticed that there was some progress with this. This is very encouraging

>>> We were missing the camera that aligned with the centre of the nozzle spigot - it is vital that this camera is aligned 100% coaxially with the nozzle spigot and plumb vertically.
if that helps, I created some cad and a 3d print to hold, what I assumed was another fid-style camera.
>>> In any event the most important item is the screen.
I was hung up on where the dot screen was located relative to the positioning dowels. Is it possible to measure that or post an image with the grid in position and a ruler showing the distances? There are a few logical positions and I couldn't figure out which made the most sense.
>>> I have set the fiducial camera using the 'prestick' method as shown to me by DC with no problems whatsoever.
Can you describe that in more detail, I would love to setup my fid camera properly?
>>> There was a lot of postulating about two sets of data points and if it could relate to the fiducial camera, and to be honest I do not believe it does.
This is my interpretation ...
Firstly, there are two clear matrices of positioning cal data in rvdata.dat. Secondly, looking at some general information about scara arms and algorithms - it seems to be a general strategy to (a) use geometry to get from desired Cartesian nozzle x,y on the bed to rotational a,b and then use (b) adjustments based on measured data to get a more accurate position. I'll see if I can locate again some articles I found on that. That points to a tactic of figuring out which square you're in on the bed and then using the cal values and 2d interpolation to get the accurate fix. Hence, you need some data showing how far off the real numbers are from the calculated ones. That points to using an accurate physical grid (i.e. the photoplot) and a cal routine to calculate the adjustments...
Yeah, since the fid camera is located a fixed distance from the nozzle it wouldn't be strictly necessary to have two adjustment matrices. However, if you've already written the cal routine code based on a camera mounted on the nozzle holder, it's perhaps the easiest course of action to just use the same code for the fid camera - particularly if they're the same camera model

Alan