I've just re-visited this cocktail (25-Sep-21) and changed the specs. to make less sour. I also eased the ginger syrup back a bit and added sugar. Previous recipe: 1½ bourbon, ¾ lemon juice, ¼ ginger syrup, ½ Lillet Blanc, and ½ pamplemouse liqueur.
I found this cocktail to be so well balanced that the individual notes were almost lost on the palate. Doubling the measure of ginger syrup helped give an identity to the drink.
Very whiskey forward but wonderful finish. I had to omit the grapefruit (can't stand the stuff) and just increased the lemon and Lillet by 1/4 to compensate.