Pellet size and payload determine exactly how much buffer you will need. You can add more, as necessary, to fill the spaces between all the pellets. Larger pellets in heavier loads will require a bit more than a lighter load with smaller pellets. Some 10 gauge loads need two scoopfuls.
Once the buffer has been poured into the shell, tap the shell and get the buffer to matriculate down into the payload. Don't overdo it, as too much effort here will displace the bottom layers of pellets, which will cause the entire payload to "float" on top of the buffer that has displaced them.
Buffered loads produce superior, more consistent patterns by reducing random pellet to pellet influences during setback. Each pellet is embedded into a protective cushion of buffer.
Industry writers are talking about BPI Buffer:
And leave it up to Ballistic Products, Inc., the shotgun people, to add a specialized Bismuth Buffer (BSB/ITX Buffer) to their excellent catalog. Tests already showed that the buffer improved patterns, but especially when bismuth was buffered.
~Sam Fadala, Blackpowder Hunting magazine