The factory presets
...well, you'll only ever hear those once at most ;-)
Is the best solution to use a VU-metre on a mixing desk to create the presets?
Seems likely... but no, because a VU meter won't tell you how loud it sounds. You'd end up with the distorted sounds being much louder than the clean ones. (A bit like the factory presets!)
I sometime uses the clip led of my jamman but this not precise enough.
and has the same problem as VUs You have to use your ears. It'll be easier to get consistent level by ear if you're playing along to a backing (or just some track). That's because level is more critical when a sound is heard in a mix. Spend more time with the levels on distorted/compressed patches, because you won't be able to fix things easy by playing quieter/louder.
As the amp has also its own amp modelling, using the ear is not enough to detect if thedistorsion come from the amps sim on the G5, the input overloaded or the amp sim on the amp.
Well, if the amp isn't overloading on a clean patch, it won't overload on a distorted patch that sound the same volume.
hope that helps some andy