The drive is 320. When formatted, the size is 297. Of the 297, about 147 is free and unused.
Nothing is missing.
You need to right click C and go to the partition menu in PW and change the "unallocated space after" dropdown to the size you want to shrink C by.
Try 50000. That's 50 GB.
Look at "option 3" in the link in post 12. It should give you some major clues in how to shrink C.