For the chipotle salt: Mix together the salt and chipotle powder on a plate.
For the margarita: Fill a cocktail shaker with ice. Add the grapefruit juice, tequila, orange liqueur and chipotle powder. Cover and shake until mixed and chilled, about 30 seconds. (In general, the drink is ready by the time the shaker mists up.)
Moisten the rim of a chilled rocks glass with one of the grapefruit wedges and press the rim into the chipotle salt. Strain the margarita into the prepared glass and garnish with the remaining grapefruit wedge.