There are a few different ways to balance your levels. One way to get close would be to look in the status tab inside the utility menu and get yourself a good base line level with that. The bad part is that clean and heavy patches don't sound the same loudness with metering like that so you'd probably be best to try to match the rest to that by ear. The important thing to me is to get a good solid reference level, match them and then have the output knob available as a single point of adjustment when conditions change.
As for how to adjust the patches themselves I'm a big fan of using the output mixer master level. If I've got a patch sounding right and all that I want changed is the volume I think that this is the safest way to ensure that I don't end up mucking up any gain stages or creating issues if I bypass an effect or something.
An even neater (and much faster way to do this) is to use the IA method where you basically program 2 footswitches, one for inc and one for dec. You can go through your patches one at a time and play them while adjusting the levels by tapping these two footswitches and the changes are saved automatically. Once you do it this way you'll see just how insanely fast you can get your patches matched and it makes a HUGE difference in how good the thing sounds IMHO. I think that there are a lot of people who don't bother to do this and are missing out.
I don't think that using a 'normal' dB setting for each patch is a good way to go about it because in my experience patches can be all over the place depending on how they are made.
