i removed the burr with the idea that i might rotate it a third to see if it fitted any better. out of the three screw holes in the burr, one does appear slightly out - but there there is enough play in the fitting that it makes no difference.
i cleaned things up and put it back, rotating the burr one third and aligning by feel against the carrier as best i could. this resulted in the lower burr looking much better aligned. not perfect, but much better.
i then re-checked all the sweet spots. the original was still the best, but the calibration had now shifted to about -4, so 4 marks closer than before! Result.
looking at my typical grind range that would put my results in the range of 2 to 9 in the extremes, and probably 4-7 for most.
