The +3 scale is from standard mount (zero) to forward 3 cm. A "center mount" on Public Enemy's is about a +7.5 cm. Just because it is more forward than the scale does not mean it cannot be done, it just has to be measured.
I can tell you honestly, the majority of people skiing PE's will be mounted between standard (zero) and +3. There is really no need to mount a PE behind the standard (zero) mount.
For all-mountain skiing, you should consider between standard and +3. The more forward you go with the mount the more forward you will want to be in your stance. If you are looking for more park specific mount, most people will go to +5 to true center (about +7.5cm). These mount positions will sacrifice some all-mountain skiing, but will ski very well switch and will have a more balanced swing weight.