Hi Veronica, I have a slightly different (I suspect older) version of this still.
Maybe I just lacked the skill, but I couldn't control reflux with cooling water flow rate.
I would definitely follow Stig's advice and buy a power controller.
One like this:
https://5stardistilling.com/product/pow ... 00w-10amp/or similar is available from a few of the local homebrew stores in Perth. Even if you upgrade to another still (as I have recently), you'll still probably use the same controller with it.