I’ve gotten noticeable but not aggressive flavor at as little as 5%. That was in a saison. In a porter I might think about 10-15%.
This was unmalted, plain buckwheat.
If you have Indian/Asian/Latin grocery stores in your area you should find buckwheat hidden away somewhere for a good price. Usually at Asian or Latin stores you’ll find it among their Indian ingredients. They’ll only carry unmalted buckwheat though.
If you just care about flavor contribution you could throw it in the mash along with the other grains. If you also want conversion then you’ll want to do a cereal mash or at least give the buckwheat a good long boil.
I’ll probably use unmalted buckwheat as I can get it locally. If I order it, I’ll have to pay shipping. However, if someone could convince me that malted buckwheat would provide more flavor, I would use it.