Go with your ear first. If you’re trying to adjust volume so it’s the same from one preset to the next, the easiest thing to do is to add a gain block to the end of the chain of each preset, tweak accordingly, and save.
probably the most important factor is adjusting these settings at performance volume. If you do it at home/thru headphones the results can be radically different when you get to the gig. If at all possible, recreate the volume level and speaker setup you’ll be jamming thru.
Are you using an Expression Pedal? If so, you could assign it to the gain of output block of the presets you’ll be using and be able to adjust your main volume level (per preset) on the fly at the gig.
I have found it helpful to put on a track, either a song on a DAW or an existing song on Apple Music/Spotify. I play that through a mixer or thought my control room monitors. Then I connect my QC to the same. either the mixer or control room monitors. Then I play along. Switching between presets and leveling them as I go. That gives me a good reference for where they really need to be because they are in context of other musicians playing with me.
I find that shows me really quickly when the clean tones are too quiet and my lead tone is so loud it will be making the front of house guy jump for the fader cuz I’m blowing something up.
You can download a dB meter app to your phone, the hand-held dB meters are also available and inexpensive on Amazon. Doesn’t hurt to do a sanity check with a dB meter as a general guide for leveling things up and then following up by using your ears. Or you can reverse that order if you prefer and go from ears to dB meter.
As other have said, if possible, the most helpful thing is listening to your presets on equipment and at a volume that will most closely approximate what you will be playing through at your performance (Fletcher-Munson). Then go make final adjustments while playing with your band if you have the chance.
Rehearsal usually provides the best opportunity for previewing presets at close to performance levels and is a great opportunity to finalize your tone for performance. Another advantage of making leveling (and EQ) changes in rehearsal is you get to hear your preset in the mix and perhaps with the specific song the preset is targeted towards, while everyone is cranked up. This can reveal volume leveling issues, but also expose problems with EQ, e.g. tone not cutting through, when played with the other members of the band.
Also, the looper is your friend. Record a loop and you can cycle through presets with the loop on and see if levels are jumping around.
I agree with Sleiweke’s method. A db meter only shows “actual” volume, primarily peaks, and not “perceived” volume in a mix. The correct volume is always a bit of a moving target as it’s dependent on how many other instruments are competing for the sonic space you are occupying,. That’s why, for me, having a master volume pedal (all presets/scenes) is a must for adjusting on-the-fly.
Also, the amount of volume boost you’ll need for solos is dependent on the volume you’re band is playing at and even how much ambient noise there is the room. At low volume, like practicing at home, a 2 db boost may be enough. On a loud stage, a 6-10 db boost may be needed for solos to cut through a loud, dense mix. I built a global lead volume control that sets the boost level of all of my lead presets and scenes globally and in conjunction with my volume pedal.
Getting in the weeds here…a loudness meter may be helpfull for balancing your presets but I’ve never tried it as I’ve had good results playing along with prerecorded tracks while changing presets then doing a bit of tweaking at rehersals and gigs.