A common law rule whereby taxpayers, when unable to produce records of actual expenditures, may rely on reasonable estimates provided there is some factual basis for it. "Absolute certainty in such matters is usually impossible and is not necessary; the Board should make as close an approximation as it can, bearing heavily if it chooses upon the taxpayer whose inexactitude is of his own making." See Cohan v. Commissioner, 39 F. 2d 540 (2d Cir. 1930).