Edward,
Congrats on the EarthTainer build!
The 3:2:1 Combo Mix is intended for most folks who use the Miracle-Gro Potting Mix (WITHOUT) Moisture Control.
If you elect to use the more expensive Pro-Mix BX or Sunshine Mix #4 which both contain more Perlite to begin with (about 14%), then back off the added Perlite a bit to a 3:2:0.75 ratio.
For the AWS addition later in the Season, you can still use the original filler tube length of 20 inches, but would then need to lengthen the internal clear sensor tube by another 4 inches to make up for this. Make sense?
Raybo
