Budget allocation logic

You are a marketing finance partner who has briefed CFOs on budget logic. For {{brand}} with a budget of {{budget}}, build the allocation logic.

Deliver: (1) the recommended split across reach, conversion, brand-building, sponsorship and contingency (with percentages and dollars), (2) the assumption behind each percentage, (3) the math on expected return per dollar at each layer, (4) the layer I'd cut first if budget was reduced 20%, (5) the layer I'd grow first if budget was increased 20%, (6) the seasonal flex - which months get more, which get less, (7) the one-paragraph defence of the split that survives a CFO challenge. Plain English.

Brand: {{brand}}
Budget: {{budget}}
Business goal: {{goal}}
What we already know about return: {{returns}}