“Although the Tribunal’s powers are not spelt out, it is implicit that it has power either to set aside the assessment or to reduce it to the correct figure … In my view, the Tribunal, faced with a “best of their judgment” challenge, should not automatically treat it as an appeal against the assessment as such, rather than against the amount. Even if the process of assessment is found defective in some respects … the question remains whether the defect is so serious or fundamental that justice requires that the whole assessment to be set aside, or whether justice can be done simply by correcting the amount to which the Tribunal finds to be a fair figure on the evidence before it. In the latter case, the Tribunal does not require the assessment as a nullity, but should amend it accordingly.”