Start with the contract, then map the payoff

The same option label can describe a different multiplier, premium unit and settlement process. Record those terms before using an expiry formula.

CheckWrite downWhy it changes the result
ContractUnderlying, symbol, expiry, exercise styleDetermines the reference price and when the right is exercised
UnitsPremium quote, multiplier, settlement currencyOne quoted premium may apply to multiple underlying units
PositionExisting exposure and each long or short legA put, call or spread changes different parts of the payoff
OperationsMargin, fees, expiry fixing, partial fillsExpiry math alone omits path and account risk

Build scenarios before choosing a story

  1. Freeze the venue specification and copy its source date.
  2. Calculate each leg in its native quote and settlement units.
  3. Map terminal prices below, between and above the strikes.
  4. Add the underlying, premiums, fees and execution costs.
  5. Mark Greeks, margin and pre-expiry value unknown unless separately modeled.