Found this on this site of all places....
"Here is the tax code document regarding constructive sales price determination:
https://www.gpo.gov/fdsys/pkg/CFR-2006-title26-vol16/pdf/CFR-2006-title26-vol16-sec48-4216b-2.pdf
It has a modification at IRS Rev Ruling 81-226 to indicate that 60% is what to use when not regularly selling at wholesale.
“Revenue Ruling 81-226, 1981-2 C.B. 213, modifying Revenue Ruling 80-273, 1980-2 C.B. 350, provides that the constructive sale price for computing the manufacturers excise tax imposed by § 4161(a), when the articles are sold at retail by manufacturers who do not sell like articles to wholesale distributors, is 60 percent of the actual selling price.”"