This was pretty bitter. I used Avallen Calvados & Fever-Tree Indian Tonic (& the Angostura bitters). I ended up adding 2 drops of 4:1 saline to try to tone down the bitterness a bit and felt like it ended up with a better balance; apple and a mild sweetness up front followed by bitterness that lingers on the tongue making you want another sip for more of that mildly sweet apple.
OK, I hate to admit but at the ripe age of 71 I have had my first cocktail with tonic. This was outstanding. We are having a rare heat wave in San Diego and this was perfect in the evening. I will definitely try other cocktails with tonic.