I can't tell if that's a missing feature on PSP X6 (don't have it, but I doubt it) or if it's a result of having enlarged your system fonts beyond the point PSP is happy with, but that dialog should have a Preview pane at the top, expandable via the Preview option on the left, showing Before and After previews. Like so:
http://prntscr.com/yygiu3
You can then use the Before pane up there to sample the color of your gray card, and the rest should be adjusted automatically.
Another method for auto color-correction is to add a levels or curves adjustment layer, then in its Properties click on the Color button, on the right side. Alternatively, click on any of the 3 dropper buttons at the bottom of the dialog (under Colors), then in the Before pane click on a black, gray or white pixel, respectively:
https://prnt.sc/yyi457
If you want more control, this video may help:
https://youtu.be/gdP3YVlrMiM . It color matches an external subject, but the tools they show can be applied to the same image as well. One of them lets you define 3 colors to be remapped to black, gray and white.
There is also this video:
https://youtu.be/QhPteAOSe3U . IMO the first part gets needlessly too technical for defining the absolute black, gray and white points in the photo (since you used a gray card, you have your gray point, then you can use your eyes to sample the black and white). However, they also use the Curves method I mentioned above (once with black point, and once with gray point) and the Black & White Points tool presented in the 1st video above (they do it to compare the results).
