Hello
B should be kept on the cvasi-linear slope of the material of choice, below the knee point. But, before of that, the core must be chosen as a product of winding area and core area, at the very least, which, in turn, is calculated based on the power requirements of the design -- you haven't specified them.
But, considering that you have calculated these and, as a result, you have your desired magnetic core, then for your switching frequency and PL-5,7 core (sufficient for this frequency), in particular, you may choose B=0.2~0.25T in order to avoid current inrush saturation.
Good luck,
Vlad