I wonder if increasing the width of the patches dramatically to something like 10mm or even 12mm would result in more accurate readings. For instance if there are some surface imperfections, dust, scratches. wheel marks etc.. would that reduce the errors or does the iSis only scan the center of the patches.
Increasing the patch width does reduce reading error. Going from 6mm to 10mm approximately doubles the patch area read and reduces the RMS error. However, patch error from various sources in my test comparing 12mm widths against 2 charts with differently randomized patch locations resulted in significantly better results with the latter even though more overall area is read with the former. I suspect this was due to ink variance differing more when the areas read were not physically close.
Also note that I got significant, large dE increases on multiple passes with a Baryta Finestra fine art 300gsm. It is very sensitive to acquiring surface creases from the spacers.
Because these creases are just under about 1mm in width, increasing patch width does significantly improve dE consistency. Much more so on the Baryta than other papers. Another approach is adjusting the paper margins.
The changes can be quite high if the creases are in the center of the patches. I was able to effectively eliminate this by shifting the left printer margin slightly so that the creases were close to the patch transition instead of near the middle. This can be done with the patch width set at either 6 or 7 mm as the spacer separation is a multiple of both.
This is pretty much a non problem with other papers.
The "baking" approach:
I've been looking more closely at the Baryta Finestra (300gsm) luster paper's high sensitivity to color shifts from the i1iSiS guides and rollers. There was a large reduction in color changes due to tracking creases by first "baking" the target prints at 120F for 2 hours. This seems to harden the printed surface with dE reductions from pass creases reduced to that of standard glossy prints which is a negligible level. However, there is a slight cost. Patch color overall is shifted very slightly about .15 dE compared to just several days of drying at room temp.
Also, the Baryta is by far the most sensitive to these guide induced color shifts. Various other luster and semigloss papers have much less sensitivity to this even with very short drying times down to just half an hour at room temp. I don't know if this is characteristic of Baryta papers generally or that particular vendor as it's the only Baryta paper I've tested this on.