“(6) [If, on an appeal notified to the tribunal, the tribunal decides—] (a) that, . . ., the appellant is overcharged by a self-assessment; (b) that, . . ., any amounts contained in a partnership statement are excessive; or (c) that the appellant is overcharged by an assessment other than a self-assessment, the assessment or amounts shall be reduced accordingly, but otherwise the assessment or statement shall stand good.”