Assessment of non-filers may be made to the best judgement; a timely valid return withdraws assessment but interest and late fee remain. If a registered person fails to furnish the required return after notice, the proper officer may assess tax to the best of his judgement using available material and issue an assessment order within five years from the date specified for filing the relevant annual return; if the person furnishes a valid return within thirty days of service of that assessment order, the assessment is deemed withdrawn but interest and late fee liabilities continue.