>>281772260
Make sure the leg, knee, and feet bones are all near perfectly in line with each other. FinalIK doesn't solve them well if isn't right. I usually do this in CATS pose mode, rotate each leg, knee and foot bone in front orthographic, then rotate the feet so they're flat, then apply as rest pose. Check all lower body bones in edit mode to be sure all of them still have a roll of zero, if not, set to zero.