The buffer dipper will help you get the right amount of buffer for every load. 1 scoop (approximately 20 gr.) is just right for most 12 gauge loads.
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, causing the entire payload to "float" on top of the buffer that has displaced them.
Buffered loads produced superior, more consistent patterns by reducing random pellet to pellet influences during setback. Each pellet is "bedded" into a protective cushion of buffer.
See buffered load data in BP loading manuals, including Advantages.