Sure there is:
Michael Mrochen published something on that, but also Antonio Guirao, or Damien Gatinel and independently myself.
You can read from my publications These:
de Ortueta D, Arba Mosquera S. Mathematical properties of asphericity: a method to calculate with asphericities. J Refract Surg; 2008; 24: 119-121
Arbelaez MC, Vidal C, Arba Mosquera S. Clinical outcomes of corneal wavefront customized ablation strategies with SCHWIND CAM in LASIK treatments. Ophthalmic Physiol Opt;. 2009; 29: 487-496
Arba Mosquera S, de Ortueta D. Analysis of optimized profiles for ‘aberration-free’ refractive surgery. Ophthalmic Physiol Opt;. 2009; 29: 535-548
Arba-Mosquera S, Merayo-Lloves J, de Ortueta D. Asphericity analysis using corneal wavefront and topographic meridional fits. Journal of biomedical optics 2010;15(2):028003
Arba Mosquera S, de Ortueta D. Correlation Among Ocular Spherical Aberration, Corneal Spherical Aberration, and Corneal Asphericity Before and After LASIK for Myopic Astigmatism with the SCHWIND Amaris Platform. J Refract Surg. 2011 Jun;27(6):434-43
anyway the Equations you are asking for are:
To my knowledge it 1st order spherical aberration (Z(4,0) / Z8) is not enough for representing aspherics which can be described by an aspheric coefficient (Q) or a conic constant. You need at least Z(2,0) or Z3 (defocus). Using Z(4,0) and Z(2,0) only you will get a rough approximation. You would need at least 2nd and 3rd order spherical aberration to calculate Q or accurately.
See also a publication from Tim Schwiegerling et al.:
"The effects of Radial Keratotomy on the Asphericity of the Cornea"
Please note that the conic constant k is equivalent to Q (refer also to publication from Antonio Calossi). There are several definitions of the conic constant which may be confusing. You wil havbe to check which one applies.
http://www.eyes.arizona.edu/Research/Faculty/Miller/Pubs/The%20effects%20of%20radial%20keratotomy%20on%20the%20asphericity.pdf
Article Corneal Asphericity and Spherical Aberration