AL = Ae Uo / Lgap, N^2 x AL = Lpri
It is necessary to have a Lpri and a peak current large enough to get the desired power, so Preq = 0.5 Lpri Ipk^2 freq. for a fully discontinuous design also di/dt = V/L so L must be low enough at the min Vin to ramp the current up to the desired settings, by setting L = v.dt / di and putting this into the power equation you can solve for Ipk and L exactly (there is only one solution). From L and Ipk you can get the gap size, AL and Npri, easy....
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Bmax should be <160mT for 100kHz for a low loss efficient design.