Dr. Marcel Leutenegger developed a model for this purpose. Check his website here. This website also include a Matlab code for Focus field calculations. The figure shown above is simply taken from Dr. Marcel's website.
Dr. Marcel Leutenegger also freely provide Matlab Toolbox for speeding up Matlabe code under the GNU Lesser General Public License. Check here for more details.
Dr. Marcel Leutenegger also freely provide Matlab Toolbox for speeding up Matlabe code under the GNU Lesser General Public License. Check here for more details.