Hi there. The best potting mix is usually determined by a couple of factors:
1. the watering schedule of the orchid grower
2. the type of orchid being grown
For a brassia, treat it as an oncidium. Oncidiums have p-bulbs to hold water, so they will like their feet to dry off. They do not want to stay dry for extended periods of time though.
I suggest, based on my watering schedule (2 times a week in the summer/ 1 time a week in the winter) that you use a free draining mix made of the following:
medium sized redwood bark
perlite (sponge rock)
medium coconut husk (for water retaining)
Pot in a pot just larger than the brassia and expect to repot within two years. Brassias will grow air roots and step out of their pots pretty quickly as they grow quickly. Depending on the size of your plant, you may have a plant that you can divide upon the next potting. I hope this helps.