Ed - I have a mix spag, bark, coconut chips, perlite and diatomite in the pot.
Then I fill up with spag around the base of new growths and keep it moist so the new roots grow well.
That is gorgeous...!!! Btw, your callosum seems to have very beautiful and clear leaf pattern... Most callosum I've seen have rather blur 'marbling' on the leaves...
looks awesome even with just 4 flowers. It's the same fine specimen as your Ho Chi Minh, way to go. I'm looking forward to seeing more of such well grown plants.
Best regards from Germany, GuRu