Raw math starts by using the formula for reactive impedance:
XL = 2 * Pi * f * L
Say you wanted 10A max at 350V. This says you want impedance XL to be 35 ohms. Your spec frequency is 100 kHz. Therefore you solve the above equation for L.
The result is approximate since it does not take into account phase changes, nor does it take into account the secondary side of your circuit.
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Nor does it consider the waveform going through the transformer (whether sine wave, square wave, etc.). It is merely a crude calculation via raw math.