Etching - acid

How much do you dilute your acids? For small projects how much acid
do you add to small quantities of water (e.g. teaspoon to 100ml
water)? Do you simply mix to saturatation? Do you buy all of these
in powder/crystal form?

Silver    - Ferric nitrate
             - Nitric acid
Copper - Ferric chloride

tx,
Regis