I have read many papers about shading and many textbooks and handbooks in PV systems. No reference has ever given a clear procedure for such a problem. Are there any rules of thumb for this?
I have studied exactly this (for two different kind of climates). See this paper: https://www.researchgate.net/publication/275244592_Mutual_Shading_of_PV_Modules_on_Flat_Roofs_A_Parametric_Study
Article Mutual Shading of PV Modules on Flat Roofs: A Parametric Study