according to my experience of winding smps transformer whether it is flyback, forward or half Bridge
powerEsim is the best tool that tells us about the "core Area, air gape, numbers of turns of primary,
feedback & secondary winding, inductance & switching frequency of the power FET". but a little trial & error may be required to correctly match the frequency
of the oscillator with the designed transformer because of the sometimes the unavailability of the mentioned core within the tool.