I have tried this ratio of 50:100 with Fever Tree indian tonic, which is a bit drier (like Schweppes dry) and even stronger in bitterness and that was a little too punchy, so I'd recommend 40:100 if you use dry tonic or with any tonic if it's your first time drinking this.
FWIW, the Becherovka website (https://www.becherovka.com/en/cocktails/original-beton/) uses 40 ml Becherovka & 100 ml Tonic, so that would be 150 ml Tonic for 60 ml Becherovka... They're also using lemon for the garnish, instead of lime, and they claim that this *is* the Czech national cocktail.