I don’t mind sharing, but mostly it comes from trial, error & practice. Lots of studying how people do things in all sorts of books & reading everything I can on the internet, and trying those methods in Seamly using the tools available, variables and formulas.
I did a short tutorial on armhole curves in this topic. I use pretty much the same methods with the sleeve cap and any other curves that I need. Have a look at it & let me know what you think and then we can discuss another tutorial on the sleeve cap.