Buck Converter Switching Frequency
There's no formula that you can use to get some "correct" switching frequency, only some guidelines.

My approach is to use the lowest switching frequency that meets all of the other requirements.  The reason for this is to minimize switching losses, EMI, driver requirements, etc. and to maximize ease of PCB layout.

One primary requirement is usually size.  The higher the frequency the smaller the magnetics, so size can be a big constraint.

Most designs these days are well above the audio range in CCM mode, but 25kHz might be a good lower bound.  I don't go below 100kHz for a simple buck without good reason.

So start by picking an inductor that meets your output current requirement and size requirement.  

Then figure out the switching frequency you need for maybe &lt;40% ripple current in the inductor, or whatever you need to meet your 1% output voltage ripple given your amount and type of output capacitance.