R1 — Build a customer-facing calculator only when pricing is genuinely formula-driven and buyers comparison-shop

Rule: Recommend a customer-facing calculator ONLY when (a) the client's pricing is genuinely formula-driven (square footage, distance, units, tiers), (b) buyers in that category already comparison-shop on price, AND (c) the client can commit to keeping the inputs current. If any one is false, recommend a directional range with disclaimers, or no calculator at all.

Why: The honest decision tree — buyers ask "what will this cost me?" (Mortgage / lending: payment, amortization, affordability calculators are described as the most-visited interactive tool on bank/credit-union sites), formula-driven pricing produces calculators that match reality, comparison-shoppers want to self-qualify (Pro-transparency counter-view: a public estimator can disqualify unprofitable enquiries, raising the quality of those who do contact you). Without formula-driven pricing the calculator either oversimplifies and anchors wrong (Anchoring effect (Tversky & Kahneman, 1974) — the first number presented becomes a reference point that pulls all later judgments, even when arbitrary) or invites legal exposure (A detailed online estimate with no reasonable basis can expose you to misrepresentation or negligence — even when labelled "estimate").

How to apply:

• Before scoping a calculator project, run the three-part test out loud with the client.
• If any test fails, propose R2 — Default to a directional range, ungated, with a loud "this is a ballpark" — not a precise gated quote instead.
• For service businesses competing on craft (Candid, most design / legal / consulting), almost always recommend the directional-range variant.