The 3:2:1 Combo Mix just applies to Potting Mixes including Sta-Green and Miracle-Gro. Pro-Mix BX already contains 14% Perlite, so when using Pro-Mix, I would do something like a 8:5:1 ratio of the Pro-Mix, Decorative Groundcover Bark, and Perlite, in that order. You want to end up in the range of 16% to 20% Perlite in the final Mix.
Raybo
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