What callout period are you talking about? I'm not referring to the last 12 hours of one's LC. Obviously, if you're not released for your last 12 hours before an off day, those 12 hours can't be considered rest. However you will still get over 30 hours of rest with zero obligation from a single day off because you can't get an assignment prior to 10AM or Noon. So you'll always get 34 or 36 hours of rest with a single day off, regardless of if scheduling marks it R or not.