The rule of thumb is equal parts fat and flour. Amount of liquid is according to how thick you like your gravy.
The secret in the taste of the gravy is the browning of the flour and the pepper, and making sure you don't get too much salt, but enough and not burning the flour mixture. If you do not brown the flour mixture enough, it is just flour and liquid, so make sure you brown it good.
Note: You can use all Milk instead of 1/2 and 1/2, but DON'T Use just all water. And use 2% or Whole Milk.
Here in the south, this is a staple food. We eat Milk Gravy (we just call it gravy or flour gravy) on and with Home Made Biscuits, Toast, Mashed Potatoes, Fried Chicken, Fried Floured Round Steak, Fried Squirrel, Fried Rabbit, Fried Potatoes. In other words if we kill it, we Fry it and we make and eat this Milk Gravy from the Grease we have left.
When we cook a Country Breakfast, and it may even be for Supper, we always include this gravy along with Fried Meat, eggs, Fried Potatoes, Homemade Biscuits, Butter, Jelly.
It is hard to tell you how much grease, flour and milk and water to use, because we just make it by feel.