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There is no ONE method or euqation, since aircraft are engineered for different tasks.

To select the proper intake or exit nozzle, the primary mathematical expression is the first law of thermodynamics, which is the conservation of energy. Air acts a lot differently at subsonic speeds than at supersonic speeds, and it acts differently at hypersonic speeds. For example, at subsonic speeds, air is assumed to be incompressible, but at supersonic and hypersonic speeds, air becomes a compressible fluid. Conservation of energy is then used to examine which type of nozzle, mostly converging versus converging-diverging nozzles, should be used.