General Potting Mix?

[Colunio]

I have been reading a number of books and online articles concerning what a good general potting mix could be made up from. One in particular, that I am thinking of using as a newcomer to growing orchids, is made up as follows:

6 Parts mixed size Bark
1 Part Horticultural Charcoal
1 Part Perlite or Pumice

Does this sound OK? If not, any recommendations as for a general mix would be very much appreciated.