I don't know what (spectral data) info comes with the basICColor greycard, but according to the PDF on their website the a* and b* values are not zero (which would be unlikely anyway, there is bound to be some variation). They mention that under the D50 illuminant a=-0.18 and b=-0.41. Besides, these values vary with different illuminants, and a single coordinate doesn't tell the whole story.
@Cliff, I've just remeasured one of my WhiBals, it's L=72.0, a=-0.1, and b=0.8 (+/- 0.1 across the surface, under a D50 illumination, 2 degree observer regime).
Thanks. Here is a summary for future reference:
Basiccolor -0.18 -0.41 0.45
Whibal 72.0 -0.10 -0.80 0.81
X-Rite 81.3 -0.18 0.28 0.31
The C* column is CIELAB Chroma, a combined measure of a* and b*.
@Dinarius - according to the Robin Myers report I linked to previously, the target values for the X-Rite in ProPhotoRGB are (190, 190, 190), and eciRGBv2 are (205, 205, 205). Since all RGB numbers are equal in these D50 spaces, maybe one of these spaces will be more convenient for doing a one-click balance.