I didn't mean to suggestion a "forms" based rule... only pointed to that concept as a way to get a handle.

The final regs provide a great and workable instruction, I think,

"...for purposes of section 199A, deductions such as the deductible portion of the tax on self-employment income under section 164(f), the self-employed health insurance deduction under section 162(l), and the deduction for contributions to qualified retirement plans under section 404 are considered attributable to a trade or business to the extent that the individual’s gross income from the trade or business is taken into account in calculating the allowable deduction, on a proportionate basis."

I think the above language, which appears a couple of places in final regs makes it pretty easy to pick and choose what we need to watch for...