Deduction mechanics for an unincorporated partnership

Suppose you have no common law employees and 3 unequal partners, and the theoretical "cost" for each of the partners is $50,000, $75,000, and $150,000. I'm not a DB person, but I seem to remember from a VERY distant past that the "default" is that the total cost is allocated to each partner in proportion the her/his partnership interest, but that this can be modified if there is a special allocation formula in the partnership agreement that provides a different result.

Is that still true (if indeed it was ever true)? And if true, can it be modified each year as necessary due to changing demographics?

Muchas Gracias.

yo creo que si, it requires modifications on the partnership agreement level.

“A partner’s distributive share of any item or class of items of income, gain, loss, deduction, or credit of the partnership shall be determined by the partnership agreement, unless otherwise provided by section 704 and paragraphs (b) through (e) of this section.”

26 C.F.R. § 1.704-1(a) https://www.ecfr.gov/current/title-26/part-1/section-1.704-1#p-1.704-1(a).

A partnership agreement of a professional-services firm, especially an accounting or law firm, often includes allocations with formulas designed so an allocation regarding the firm’s pension expense approximates the expense attributable to each individual partner.

For pension expense attributable to people other than partners and their beneficiaries, an allocation might be general regarding a whole firm or a whole department of a firm, or might be particular regarding those associates and other employees accounted for in the partner’s cost structure.

I think as soon as you mention to a CPA that you're inclined to reduce K-1 income by splitting by the total cost by the partnership ratio (rather than along the lines of who actually got what among those partners), they tend to indicate the partners have something along those lines in place to better align the deductions for each.  Like their DC plan would.