Typically, during the sound check before a concert, the guitar level in the mix will be set my the sound engineer at the desk. Whilst it's the guitarists turn, he will ask for the 'normal' guitar level, and then the 'Fx' level, and may ask for an adjustment of the balance between the two. It there's no engineer, or if there's no guitar through the PA, it's up to the guitarist (and the band as a whole...) to sort out the relevant levels before the concert. There is no absolute rule, but, in general, fuzz, distortion, overdrive etc need not be so loud as one might imagine. Basically one would aim at having the perceived 'loudness' pretty much equal between clean and fuzz. Once the levels are set for the band, for the venue and for the song, it shouldn't need tweaking at all. If there's a lot of boosting going on for some songs, a second pedal, after the fuzz, to be able to 'up' the level a little might be a Good Idea. If there's even more going on, a programmable Fx becomes useful, and learning how to 'tap-dance' for switching between patches does the job. As you see, there's no 'silver bullet'; start with the simple way (get the levels about equal...) and work up from there by experience, listening to recordings of your rehearsal, for instance.
Hope this helps...
Douglas