"The element of guess-work and the almost unavoidable inaccuracy in a properly made best of judgment assessment, as the cases have established, do not serve to displace the validity of the assessments, which are prima facie right and remain right until the taxpayer shows that they are wrong and also shows positively what corrections should be made in order to make the assessments right or more nearly right." ( Bi-Flex Caribbean Ltd v Board of Inland Revenue (1990) 63 TC 515, 522-3 PC per Lord Lowry). That was confirmed by this court, after a detailed review of the authorities, in Customs and Excise Commissioners v Pegasus Birds Ltd[2004] STC 1509 ; [ 2004] EWCA Civ 1015 . We also cautioned against allowing such an appeal routinely to become an investigation of the bona fides or rationality of the "best of judgment" assessment made by Customs: "
“[10] In Rahman (t/a Khayam Restaurant) v Customs and Excise Comrs[1998] STC 826 (' Rahman 1 '), Carnwath J (as he then was) stated that a tribunal should not treat an assessment as invalid merely because the members disagreed as to how the commissioners' judgment should have been exercised. A much stronger finding was required, for example that the assessments had been reached dishonestly or vindictively or capriciously, or was a spurious estimate or guess in which all elements of judgment were missing or was wholly unreasonable. [11] The principles established in Van Boeckel and Rahman 1 indicate that the FTT's [First-tier Tribunal’s] jurisdiction when considering whether an assessment was raised to the best of the commissioners' judgment is akin to a supervisory, judicial review type jurisdiction. The FTT does not have a true appellate function in that it cannot set aside the assessment on the basis that it disagrees with the commissioners' decision to make the assessment. The circumstances in which the FTT can decide that the assessment was not raised to the best of the commissioners' judgment, and therefore should not have been made at all, are very limited, essentially being restricted to cases where the commissioners have acted perversely or in bad faith. Carnwath J in Rahman 1 indicated that this 'kind of case is likely to be extremely rare' and that in the normal case 'it should be assumed that the Commissioners have made an honest and genuine attempt to reach a fair assessment': see page 836 of the judgment.”