How to choose switched regulator and calculate the filtering (LC) corresponding
The datasheets contain the math that is correct for the specific part in question. 

I would note that topology matters for noise, switched cap is worse then flyback is worse then boost is worse then buck is worse then quasi resonant in general. 

Also, pushing extreme efficiency does not help with noise, you don't want the edge rates and dI/dt to be too stupidly high. 

LT have some parts specifically designed to limit EMI, but do follow the layout suggestions, layout is everything in this game.

One other tip, sometimes you can make switcher noise substantially irrelevant by synchronising the switcher with something like an ADC clock (Makes the switcher spurs alias to DC), it is a useful trick when it works.  

Do be careful with LC filters on the output, it is tempting to make L large, but that hurts both because of the slowed transient response and because the self resonance rapidly becomes painfully low, also watch the Q, you don't want much or an LC network can become horribly badly behaved at some frequencies.

Do not forget to filter the input side, especially in buck converters, but really any variety can cause problems with radiation from the input side power traces.