@NiveusLepus By eye! It's about as much as you'd use to spread on two slices of toast.
It's an old-fashioned term and is generally equivalent to 2-3 tablespoons or 20-30grams. It is intentionally imprecise, so you have wiggle room.
I agree @tyghebright, its fun to work it out
In my understanding a knob of butter is about the size of tip of your pinky
@stueytheround That’s interesting and a touch subjective.
I’m absolutely intrigued .