Yes I too wonder why so many people fret over %'s of this and that and this and that and some more of this....

Especially when you can get killer results with simple Pro Mix or Scotts Potting soil.

Good Question.

Look for the smallest lave rock you can find and it must be the RED colored kind as it contains nice amounts of iron.

At least one inch layered at the bottom...ideally several more if your container can hold it AND good amounts of soil.

Certainly use enuff to cover about the drainage holes on the sides.

In a 5 gallon bucket, I like to use about 3-4" worth.