Compute the reference strain using Equation (6).

def _calc_strain_ref(x_3, x_3_mean):
        """Compute the reference strain using Equation (6)."""
        b_9 = -1.41
        b_10 = -0.950
        return np.exp(b_9 + b_10 * (x_3 - x_3_mean)) / 100