Honestly, your phrag is going to be pretty adaptable to most any standard orchid potting mix. The biggest key to success is going to be to keep the plant's roots wet at all times.
How big is your plant, in terms of # of growths and current pot size? The bigger the plant, the bigger the pot, which generally means you'll want to select larger grade potting media to help ensure that the entire root mass stays fresh and aerated as possible.
Personally, I grow almost all my slippers in a rockwool based mix, and I've had amazing results with it. But I also add quite a bit of medium-large grade charcoal and sponge rock, plus some LECA (s.a. hydroton). Then I sort of season to taste with chopped sphag, sand, eggshell, oyster shell, etc. Considering the parentage of your plant, if you go with a mix like this, it seems to help to have a decent calcium/buffering source like oyster shell, crushed coral, limestone chips, things of that nature.
Nearly all my phrags are in transparent plastic pots, so even if the roots didn't quickly push out of the top and bottom of the pots as they frequently do after running out of space around the perimeter of the pot, I can see that they're nice, active, and healthy.
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