I made this according to the recipe and thought the amount of ginger beer was perfect. I was definitely looking for the mezcal to shine though - the ginger beer was the supporting act!
I've only tried one Mezcal so far: Del Maguey Vida de Muertos Mezcal Artesanal. I don't know if that one is very potent compared to others, but it completely overpowered Fever-Tree's ginger beer, which is amongst the strongest flavoured ones you can get..
An extra 2 cl of ginger beer made a better balance in my case.
I used exactly the same Mezcal, and also the same ginger beer. Even though I measured all the ingredients, I eyeballed the ginger beer. Ended up using around 100 ml so around 3 and 1/3 ounce. It turned out delicious, however the colour wasn't very appetising