Chapter 2: An I/F Primer is an introduction to flux, intensity and reflectance intended to clearly present this often obscure topic. The Primer also discusses the plane-parallel single-scattering formulae of Chandrasekhar, with planetary rings as examples.
Appendix A: Saturnshine contains the only (to my knowledge) publically-available detailed discussion of the calculation of the contribution to the reflectance of the rings due to sunlight scattered by Saturn onto the rings.