First time posting, hi everyone!
I had this exact problem too, I first tried a decibel meter on my phone but then I stumbled across this bloke’s video on youtube. He uses a free DAW plugin called called MLoudness Analyser. I downloaded it and used it through my DAW (Reaper in my case but I guess it’ll work with the any DAW).
I just followed what the bloke said in the video and it then gave me a great starting point by allowing me to adjust and save every preset and snapshot to pretty much the same final volume, then I just needed a to make a few minor adjustments by ear next time I practiced with the band and then of course did the final save on any presets/snapshots that I’d had to tweak.
From then on, whenever I’ve created a new preset at home I’ve done it with Reaper on the laptop hooked up to the Helix from the start, so I know I’ve got that ball-park starting volume, whichever Helix amp I choose, before making the final volume tweaks by ear next time I practice with the band.