Best judgment assessment must be reasoned, not arbitrary; it requires material support and more than mere guesswork.
A best-judgement assessment allows limited estimation but the assessing officer must make an honest, fair and reasoned estimate and cannot act wholly arbitrarily; technical rules of evidence are relaxed but the assessment must be based on more than mere suspicion or pure guesswork and should be supported by adequate material rather than unsupported conjecture.

Best judgment assessment: courts may not substitute their own estimate if the assessing authority's basis has reasonable nexus.
Assessment based on accounts is proper where books are genuine and substantially correct, with only minor adjustments; a best judgment assessment is used when accounts are unreliable and the authority estimates liability using available accounts, other information and surrounding circumstances. Courts reviewing a best judgment assessment must first confirm that rejection of accounts was justified and then assess whether the estimating basis has a reasonable nexus to the estimated turnover; if so, the authority's bona fide estimate should not be displaced.

Assessment in service tax: scope includes self assessment, reassessment, provisional and best judgement modes and interest determination.
Assessment for service tax includes self-assessment, reassessment, provisional assessment, best judgement assessment and any order where tax assessed is nil; it also includes determination of interest on assessed or reassessed tax. "Assessee" means a person liable to pay the tax and includes the person's agent.

Untrue self-declaration in tax return corroborates suppression and can trigger penalty under self-assessment procedures.
An untrue declaration in a service tax return asserting that tax has been paid corroborates suppression and attracts penalty; absence of a bona fide statement on the return or with the return renders the declaration faulty and imputes liability under the self-assessment procedure.