De-embedding of metamaterial unit cell for normal incidance

I am trying to redo the paper Electric-field coupled resonators for negative permittivity metamaterial by Smith.But my problem is that for the case of normal incidence,i do not know how to choose proper de-embedding distance for correct S21 parameter phase calculation.If phase changes extracted parameter values will change.So i am confused.Any suggestion will be of great help.

Thanks

Take positive distance between substrate and Port plane as a de-embedding distance to get correct phase of s21.