selection of common mode choke
This answer assumes you are connecting your power supply to a household AC source. I have no specific answer for just a DC in and DC out buck regulator because, unless this is say a 48V telecom application there will be the strong possibility of no spec existing. If a spec exists then fine!

Whatever country you are in there are pretty similar standards to be met so concentrate on these standards first and, in particular concentrate on conducted emissions. Understand what levels of emission are allowed to be produced. Not knowing what you aim for gives you zero chance of theoretically selecting an inductor.

So, let's say you have the numbers (limits imposed) and of course they will vary across the full range of conducted emissions so what I'd do next is use a simulator to mimic the switching currents and try out different scenarios with simulated common-mode chokes and capacitors on the AC side.

The inductor/choke and the capacitors form a low pass filter (significantly below the switching frequency) and you can see how much attenuation you get from the simulator.

Sorry, I can't answer this for just a buck regulator unless there are specifications at hand. If there are specs available then follow the same guidelines. This won't guarantee a "pass" but it avoids a miserable and costly "fail".

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