I think the Prism software is doing its job well and the results will only be as good as the target and its calibration file. The status of Faust is now iffy at best, but they seemed to have produced good results in the past. The previously mentioned UK supplier is very responsive, but I question the accuracy of their data files. It would seem the best answer is a scan of the specific target you're using and that would appear to be available here- https://www.chromix.com/colorgear/product.cxsa?toolid=434
IT-8 targets aren't that easy to get. The Silverfast/Lasersoft target from B&H appears to be a new style target and I suspect that's not compatible with Prism unless Mike says otherwise.
https://www.bhphotovideo.com/c/product/1667496-REG/lasersoft_imaging_la1208_silverfast_3_93_x_5_90.html
More in the distant future if I can get both my old and new targets rescanned. At this point I think it would be cheaper if I just bought the extra large 152-count box of Crayolas and made my own. :-)