Best judgment assessment: assessing officer may determine taxable income after considering material and granting opportunity to be heard. Best judgment assessment allows the Assessing Officer to determine total income or loss when an assessee fails to file required returns or comply with specified notices; the Assessing Officer must consider all relevant material and, after serving a show-cause notice giving the assessee an opportunity to be heard on a specified date and time, make the assessment to the best of his judgment. A transitional provision preserves earlier application of the section for prior assessment years and an exception removes the hearing requirement where a prior notice under the relevant subsection was issued.