Assessment powers: officer may best-judgment assess non-filers; return within thirty days withdraws assessment and interest remains payable. If a registered taxable person fails to file the required return after notice, the proper officer may, after allowing fifteen days from notice, assess tax liability to the best judgment based on available material and issue an assessment order within prescribed time limits; a valid return furnished within thirty days of that assessment order results in the order being deemed withdrawn, without prejudice to the levy of interest or late fee.