As I understand the Cohan rule under the IRS' Guidelines For Determining Noncompliance, taxpayers are allowed a deduction for an estimated amount of expenses when it is clear the taxpayer is entitled to a deduction but is unable to establish the exact amount of the deduction. Specifically the IRS states on their web site the following.
"The “Cohan Rule,” as it is known, originated in the decision of Cohan v. Commissioner, 39 F.2d 540 (2d Cir. 1930). In Cohan, the court made an exception to the rule requiring taxpayers to substantiate their business expenses. George M. Cohan, the famous entertainer, was disallowed a deduction for travel and business expenses because he was unable to substantiate any of the expenses. The judge wrote that “absolute certainty in such matters is usually impossible and is not necessary, the Board should make as close an approximation as it can.” In general, the Tax Court has interpreted this ruling to mean that in certain situations “best estimates” are acceptable in order to approximate expenses. The Cohan Rule is a discretionary standard and can be used to support a reasonable estimate of compliance requirements."
This worked well for the taxpayers in Armando Sandoval Lua v. Commissioner TC Memo 2011-19 in that the taxpayers provided sufficient evidence demonstrating additional compensation expense was incurred for additional services provided even though it was in the form of cash.