Jack and Hg,
I almost hate to disagree, but the equipment will likely work better if you
use SMEMA standard fiducials.
1mm diameter copper with a minimum of 1mm clearance to anything all  around.
No copper, no mask, no nothing within 1mm of the fiducial dot.
Unfortunately using Sn100 finish does not allow a shiny dot for the
equipment to find, so you need the minimum clearance to help.
If you can try a coupon with multiple variations of clearance .5mm, 1mm,
1.5mm, round, square, etc.
I am relatively sure you will find the SMEMA standard of 1mm round with 1mm
round clearance the best, but the finish may have more effect and there fore
need something to help compensate.
Good luck,
FNK

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