Astigmatism is a very difficult concept to explain to students and patients. There are no good webpage references on the combination of different spherocylinder lenses. Understanding how they add is however important in understanding how the Jackson Cross Cylinder works and also in understanding how astigmatic changes come about with surgery.
At North West Eye Specialists we have the Holladay software for analysing surgical astigmatism which is allowing each of our doctors to form their own database parameters for the optimized management of astigmatism.
As a teaching tool, i have developed an online astigmatism calculator – it can add and subtract spherocylindrical lenses. It is still in beta form and not 100% accurate but will help in the understanding of astigmatism. Please feel free to leave suggestions or drop me a line for the source code