added a comment - - edited
The code responsible for the conditional checks is in lib/conditionlib.php in the function is_available(). If you go to the line "$score = $this->get_cached_grade_score($gradeitemid, $grabthelot, $userid);" and place underneath it "echo $score;" (without the quotes) the score will be printed to the screen. If you log in as a student, and then compare the score printed to the grade range specified in the settings you can see if the grade range is being met. Please note, remove the echo statement ASAP, as it will appear for all students. If you aren't comfortable with outputting the score to the browser, then feel free to write it to a file, or only output it depending on the USER->id, this will require some coding knowledge though.