Assessment of non-filers: best-judgement assessment may issue, withdrawal possible upon timely filing of a valid return. Where a registered taxable person fails to furnish the required return after service of a notice, the proper officer may make a best judgement assessment of tax liability using all relevant material and issue an assessment order within the prescribed time limit; if a valid return is furnished within thirty days of that assessment order, the order is deemed withdrawn, while liability for interest and late fee remains preserved.