John,
You are absolutely correct that the type of potting mix you use does impact the wicking action. I am not a hydrology Engineer - - just doing it by trial and error, so other experts here should be asked of their recommended mix composition. At the end of the day, filling a few 'Tainers with your mix and using a Moisture Meter to read relative moisture will be your best bet.
Just make sure that as you add 1" or 2" layers of the mix into the 'Tainer to thoroughly wet the mix, as if you get dry pockets, this will inhibit the wicking process to the plants.
Ray
