What kind of orchids are you gonna repot? (I ask because I dont use perlite with phalaenopsis, only bark. Actually I use perlite mostly with plants that go in soil, and not orchids. Do have some in only perlite, and I do have some perlite in with some seedlings)
The easiest way is to figure out the size of the pot- usually says how many liters the pot are, and you need pretty much the same amount of potting media. (A little less, roots take some space- but it will give you a clue)
If you use bark, there will be air for the roots. Adding perlite can actually make it less airy. Mixing smaller and bigger particles makes it less airy. If you use bark, the bark will leave airpockets anyway.
Best of luck with the repotting
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