I'll probably get told off for saying this, but I usually just mix
until the mixture is a dark orange colour. The strenght of the
liquid affects the etch time too, as does heat. I usually sit the
container of ferric solution in a bath of warm water to speed the
etch up a bit. I have a small coffee sized jar of the crystals,
which doesn't go far if you need to etch big objects.
So the answer is: I don't know the exact quantities either. I'm
interested to see if anyone else knows.