Assessment of non-filers: tax liability may assessed to best judgment; valid return withdraws assessment but interest and late fee remain. The proper officer may issue an Assessment Order assessing tax to the best of his judgment when a registered person fails to file required returns despite notice; the order must be issued within the prescribed limitation period from the annual return due date for the relevant year. If a valid return is filed within the short period after service of that order, the assessment is deemed withdrawn, but liability for interest and late fee arising from the failure to file continues.