Place 1 Tbsp. grapefruit juice in a shallow bowl or saucer. Place kosher salt in a separate shallow bowl or saucer. Dip rim of an old-fashioned glass in grapefruit juice in shallow bowl, and then dip damp glass rim in salt to coat the rim. Fill glass with ice. Add sparkling mineral water, tequila, simple syrup, lime juice, and 1/2 cup grapefruit juice; stir to blend. Garnish, if desired.