I've only ever grown in fabric pots, and my mediums were always amended with a good 20-30% perlite by volume. I currently use Pro-Mix HP which is a peat moss medium that contains around 30% perlite already, so I didn't add any extra. I guess it depends how much is already in it, and whether you want more or not. But for me, 20-30% perlite with the rest being non-perlite is a ratio that has worked well. I like seeing lots and lots of white in my mixes