Thanks for chiming in Roy.
Regarding the 51-step chart (which was recommended to me by those at Piezography HQ who read a 256-step target for their proprietary .quad file creation procedure) and the latest version of i1Profiler software and the i1PRO device --
The x-rite documentation states that the measurement data includes many readings of each spot which are averaged as the device is slid across the target. There is also a dual pass mode where each strip is read twice and averaged.
Given that, it seems to me that if QTR-Linearize-Quad does accept the measurement file from a 51-step target (it has for six out of the seven papers I have been testing with) wouldn't it be better to use those measurements rather than the 21x4 target?