I have a similar setup with my Helix, and it has been working well for many years. Basically I wanted to use two sets of output speakers, and to be able to control their relative volumes. I'm using a competitor to the Mackie Big Knob: The Behringer Control2USB (Monitor Controller). My setup is pretty simple:
Helix (and Other Sources) --> Mixer --> Behringer Control2USB --> [Studio Monitors] + [PA Speakers]
If you're using just the Helix as a source, you shouldn't need the mixer at all. As for the Behringer, I am using the output from the mixer, going into one input on the Monitor Controller
As rd2rk mentioned, I think you'll find that the Helix has significantly better specs (lower latency, higher bit-rate and frequency) when used for digital recording or output. In fact, I never even bothered to hookup my Monitor Controller via USB, though it does have that option. Generally, if I'm recording anything, it's direct from the Helix into a DAW. I guess if I needed to record other channels (like drums or keyboards going into my mixer), I'd use the Behringer Studio Monitor (or maybe run the other instruments through the Helix's additional inputs).
The setup works great for me: I can independently manage the volume of my monitors and PA speakers using the "big knob" (what a name!) on the Behringer. It also has handy "mute" and "dim" (lower all volumes with one button press) options. I use the Helix as the primary output of my PC and control computer sound output levels using the volume controller on my keyboard. That, along with the Helix's built-in expression pedal for guitar/bass volume, keeps all of the volume level controls within arm's reach.
It sounds like you're on the right track, but I hope this helps.