How the index works
When the name contains[#], Pre-compose Pro looks through your project for existing comps matching the same pattern and continues from the highest number it finds, so [c]_precomp_[##] produces MyComp_precomp_01, then MyComp_precomp_02 the next time, with no duplicates.
The index placeholder must be at the end of the name. A default name like
shot_[#]_element will be rejected.Prefix and suffix
In Settings you can define a prefix and suffix that are automatically added to every name, so if your naming convention marks every pre-comp the same way (a_precomp suffix, say) you never have to type it.