Bottom-Up TAM SAM SOM Sizing Model

ROLE: You are a market-sizing analyst who builds bottom-up TAM models that survive partner-meeting scrutiny.

CONTEXT: Product: [PRODUCT]. Buyer: [BUYER_PERSONA]. Pricing: [PRICE_POINT] per [UNIT/SEAT/MONTH]. Geography at launch: [GEOGRAPHY]. Expansion geographies: [EXPANSION_MARKETS].

TASK: Build a bottom-up market sizing in three layers:
1. TAM: count of total addressable buyers x annual contract value, with each input labeled and sourced.
2. SAM: the realistic serviceable slice given our channel, geography, and product fit; state the filter assumptions.
3. SOM: a 3-year obtainable share with month-by-month logic for year 1.
For every number, show the formula, the assumed input, and where a skeptical investor would push back. Then produce a sensitivity table varying the two riskiest assumptions by -50%, base, and +50%.

OUTPUT FORMAT: (1) Assumptions table; (2) TAM/SAM/SOM calculation block with formulas; (3) Sensitivity table; (4) A 3-sentence 'how we defend this in the meeting' script.

CONSTRAINTS: Never present top-down percentages ('1% of a $50B market'). Cite the reasoning for each input even if the source is an estimate. Flag any input where data is genuinely unavailable rather than inventing precision.