I have made a drink similar to a lemon drop, only w/ grapefruit.

For lemon drops, I use 1 part vodka, 1 part lemon juice, 1 part simple syrup.

For grapefruit drops, I use 1 part absolute grapefruit vodka (not liquer, vodka), 1 part grapefruit juice and 1/2 simple syrup. Sugar the rim if you like.