(b)After the evidence has been so taken, or the summary of evidence has been read, as the case may be, the accused may make a statement on mitigation of punishment, and may call witnesses as to his character. If the accused at any Court-Martial states anything in mitigation of punishment which, in the opinion of the Court, requires to be proved and would, if proved, affect the amount of punishment, the Court may permit the accused to call witnesses to prove the same;