I know gravy. My mother-in-law taught me gravy and she learned from her own mother-in-law, who learned from her mother who cooked for a living. Are my credentials clear?
There are two secrets to perfect gravy: the roux and the stock. Once you understand these two concepts the rest is easy.
You need a stock that has been separated from it's fat. To do this I like to let it all settle quietly in the fridge. Then it's easier to skim off the solid fat from the broth. If you have a full-bodied stock it is gelatinous when completely cooled and thus easy to separate. But you don't get a stock like that in 15 minutes or even a whole day.
Obviously I can't do all that on the same day I roast the turkey. My secret is to stay one turkey ahead --or, at least, a chicken or two. This Thanksgiving I used chicken stock I had frozen over the last couple of rotisserie chickens I bought for dinner. Nobody knew the difference since there was such an eating frenzy
To get this stock I take the entire left over carcass and roast it at a very low temp (maybe around 250) for about six hours in the original roasting pan with lots of water that mostly covers the bones. You can have about an inch of bones above the water. The experts try to tell you to boil this mess but you end up with a nasty looking foam that the experts tell you to skim off. Well, I don't want to spend my day skimming nasty foam. I've been roasting the carcass in the oven or years and for some reason you don't have the foam when you roast the bones in the oven instead of the stovetop. You also don't have to babysit it all day. Putting in the oven lets you forget about it. You might check a couple of times to make sure it still has enough water. Eventually the bones will just fall apart into the water. When you think it's ready (or when you're ready to go to bed), scoop out the bones and onions, carrots and celery. Give the bones to the dog. Strain what's left into a big bowl. You'll get about three or four cups, depending on how much water you put it. Set the stock in the fridge for a couple of days or so until it gets really firm. Then you skim off the fat and, voila, you have a dark brown gelatinous stock.
Once you have a good stock you're ready for the roux.
To start the roux you melt a chunk of butter then add flour. My mother-in-law always used a whole stick of butter but it was harder to see how much flour she used. I think she just added it until it looked right. You stir it around until you get a smooth (but not thick) paste then keep stirring until it browns. Then you add the stock. The reason it's in zip lock bags is so you can thaw it faster. To speed things up you can put the zip lock bag in a pot of hot water. I think I usually use about three cups. When it's all combines, heat it to a boil stirring with a wire whisk. It will thicken into the perfect gravy.
Relax. You don't have to have homemade stock. If you don't have the golden stuff I made from the bones you can just use the drippings from the pan. It would probably turn out just fine. I get compulsive sometimes.
On Friday I cooked the Thanksgiving carcass into stock. Then a couple of days later skimmed off the fat and put the stock into zip lock bags. Then I froze it for use on Christmas. We'll probably have Christmas at Elizabeth's house and I will be able to make the gravy here in my own kitchen. I can take it to Garland in either a zip-lock bag or a slow-cooker to keep it warm. My Thanksgiving gravy will never have known her Christmas turkey. But our lives will be much calmer for the effort.
I can't give dependable amounts. I learned by watching Beaven's mother and trying it myself. I'm more of an organic cook than a chemist. Come by my house the next time I make gravy and I'll show you. Give yourself permission to flop a time or two. Don't freak out if and when you mess up. You just won't have gravy that day. Gravy isn't good for you anyway.