I use a little bit different recipe I came across. It adds a quinine extract from "herbmama" out of Nevada, found on amazon. No stovetop involved. We've been using this for nearly 3 years now. No one has been sick the entire time in this house of pure-bloods.
Also, a couple of good 'tonic waters' are available: One by Fever Tree 'Indian Tonic Water' and the other is called "Q" Tonic Water.
Just an FYI.

P.S. I purchase cardamom seeds, not the pods. I use a good teaspoon full, give or take, PLUS, I use the same amount of fennel seeds and pulse them together a couple of short pulses just to break them up.
The fennel is quite high in shikimic acid which is recommended for this 'illness' and its affects. I figure it can't hurt.