With the remaining cocktail, we added a few small ice cubes. The cocktail does work great a bit bit more water.

Liked this, but found it a tad on the sweet side for our taste. We put in a few drops of grapefruit bitters and still on the sweet side. Sort of a cassis version of the Cosmopolitan.
Our grapefruits are picked ripe from the tree and tend to run sweet. Would consider using white grapefruit which is a bit more sour. I would rate this more of a "6" than "7" for sweetness.
Would be a great cocktail for a brunch or a warm afternoon.

Looking at the comments, I modified it to 60 Vodka, 50 Pink Grapefruit, 20 Cassis. It was a refreshing bitter sweet cocktail. A nice balance of the ‘tang’ of the grapefruit with the fruity cassis.

It's fine, but not sure I'd go out of my way to make one. If I've squeezed a fresh grapefruit, I probably want a drink that keeps things very grapefruit-forward, whereas here the cassis just detracts from it a bit I think.

Perfectly delicious with white grapefruit. Surprisingly mellow and not sweet despite the amount of crime de cassis. Wonderfully balanced.