It turns out if you simply open the FITS file in Prism and after saving the file in Prism and then edit the FITS header, it seems to be able to properly analyze it.
So my next question on this topic is whether this result is good or bad (see the resulting tilt & curvature images).
For tilt, is +/- .01 mm in one diagonal & +/- .06 mm in the other something that I should try to tweak?
For curvature, it looks like one diagonal is flat @ .006 mm & the other is - .05 mm in the corners. Again - is it worth chasing a fix?
Position X=1 Y=1 0.059 mm
Position X=4656 Y=1 -0.012 mm
Position X=4656 Y=3520 -0.060 mm
Position X=1 Y=3520 0.011 mm
Position X=2328 Y=1760 0.000 mm
Slope angle (°)= 0.61
Position X=1 Y=1 -0.050 mm
Position X=4656 Y=1 0.009 mm
Position X=4656 Y=3520 -0.044 mm
Position X=1 Y=3520 0.006 mm
Position X=2328 Y=1760 0.006 mm