Oss / Oeric casing mixture:

7.5 liters peat, 3.5 liters vermiculite, 4 liters fine washed sand,
2 liters calcium carbonate (finely crushed oyster shell)

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Stamets Casing (not psilocybe specific) from Growing Gourmet & Medicinal Mushrooms 1993

10 units peat
1 unit calcium sulfate (gypsum)
1 unit calcium carbonate (chalk)

Use calcium carbonate to adjust pH of peat.
Mix dry components - add water slowly, when water can be squeezed out to form brief rivulets, then proper moisture has probably been achieved.
A 75% moisture content is ideal and can be tested by measuring the moisture lost from a sample dried in a hot oven.