Best judgement: the law
When you make an assessment under Finance Act 1994 section 12(1) you must ensure that the assessment is made to best judgement. The law states:
Section 12 (1) Finance Act 1994
… they may assess the amount of VAT due from him to the best of their judgement…
Any assessments made under this power must satisfy the best judgement criteria. This means that given a set of conditions or circumstances, you must take any necessary action and produce a result that is deemed to be reasonable and not arbitrary.
In other words best judgement is not the equivalent of the best result or the most favourable conclusion. It is a reasonable process by which an assessment is successfully reached.
Although there is no reference to best judgement in other excise assessment powers, see EAIG2400 you should apply the same basic principles and make such assessments to the same standard of best judgement.