I am currently looking at isotropic properties and let's say I want to determine my property in the  direction. Now, I basically want that my  direction be the same as the  direction in an experimental setup.
If I switch between the conventional unit cell and primitive unit cell, the direction changes. It changes with respect to the symmetry of an infinite crystal lattice (the primitive cell is a diagonal part of the conventional standard).
How do I go about resolving this problem? There must be some standard to set the underlying coordinate systems, otherwise everyone would be getting confused by this.