I'm afraid it isn't very exact, but here's what I do:

1) Boil water with cinnamon (one or two sticks), cloves (1 per 2 cups or so), cardamom (I use the kind that are seeds that have been removed from pods - a spoonful, more or less depending on the amount)

2) When the water is boiling, add black tea (bags or loose). Use the amount you would in a pot - about a teaspoon per cup or a little less if it is Indian black tea

3) Simmer until it is as dark as you like

4) Add sugar and milk. Sugar - about a heaping tablespoon per two cups. Milk - usually about a quarter as much as there was water. I suggest tasting for sweetness; after a while you can tell by the amount.

5) Return to near boil - if you skimp here, it won't be quite as authentic a flavor, but it is easy to get impatient.