I often see answers for one-off designs including the advice, "Just wind a transformer...." Like it's no big deal. Did I miss a class in which they handed out a sheet listing the formulas for core size, composition, geometry; wire gauge and insulation; and winding style as a function of voltage, power, frequency, duty cycle, and load application?
But I also frequently see people scavenging and reverse-engineering the pins on flyback transformers from CRTs.
So I'm deducing that there's an envelope inside which one realistically hand-winds a custom transformer, and outside of which it would be difficult/wasteful/dangerous to do so. What are the parameters of that envelope, and why?