Flyback Transformer Primary Inductance Calculation
I want to know how the value is derived without measurement


The inductance of a flyback transformer is based on: -


Number of turns (actually it's number of turns squared that's proportional to inductance)
Core permeability (more permeability means more inductance)
Core dimensions (a shorter mean length and bigger cross sectional area increase inductance)
Core gapping (if used, L will reduce with more gap)
Operating frequency (if above a certain point L reduces due to permeability changes and, above usually a higher frequency, self resonances can make inductance measurements pointless)


And you need to assess if the peak current is causing saturation because that reduces effective inductance.