Pour pan juices through a fine-mesh sieve into a 2-quart glass measure (do not clean roasting pan), then skim off fat and reserve. (If using a fat separator, pour pan juices through sieve into separator and let stand until fat rises to top, 1 to 2 minutes. Carefully pour pan juices from separator into a 2-quart measure, and reserve fat left in separator.) If there is less than 1/2 cup reserved fat, add melted butter.
Add enough turkey stock to pan juices to total 8 cups liquid (2 quarts). Straddle roasting pan across 2 burners, then add 1 cup of remaining stock and deglaze pan by boiling over high heat, stirring and scraping up brown bits, about 1 minute. Pour through fine-mesh sieve into glass measure with stock.
Whisk together reserved fat and flour in a 4-quart heavy saucepan and cook roux over moderately low heat, whisking, 5 minutes. Add hot stock with pan juices in a stream, whisking constantly to prevent lumps, then bring to a boil, whisking. Stir in any turkey juices accumulated on platter and simmer gravy 1 minute. Season with salt and pepper.
Cooks' note: Gravy can be thickened using cornstarch instead of a roux. Discard fat from pan juices. Cool 1 cup of stock (uncovered) or bring to room temperature. Stir the 1 cup stock into 1/2 cup plus 1 tablespoon cornstarch in a bowl until cornstarch is dissolved. Pour 8 cups stock with pan juices (see recipe, above) plus deglazed pan drippings into a 4-quart heavy saucepan and heat over high heat until hot. Stir cornstarch mixture, then add to hot stock in a stream, whisking constantly. Bring gravy to a boil, whisking constantly, then stir in any turkey juices from platter and boil gravy, whisking, 1 minute. Season with salt and pepper.