I don't know if this applies directly, but when I play live I use my Helix LT to mix a monitor send with my guitar and then to wired IEMs. I've got the big knob set to just control my headphone volume and not effect any of my other outputs. You can do this in global settings.
In your case, if you did that then the volume going to your studio monitors would be fixed (do you have a level control on the back of your monitors?) and the volume going to your headphones would be controlled by the big knob.