Mostly it is not scientific or measured or calculated at all. And it doesn't cost me much time either.
I only had white wool, that needed dyeing, but I did not want a plain effect.
It is said that wool need room in the pot etc etc.
I placed a layer of wool (soaked first in water with some vinegar added) in the bottom of the empty pot.
Then sprinkled some russet red (Jaquard) powder on.