Pour the pan juices into a glass measuring cup and skim off the fat. Spoon 3 tablespoons of the fat into the roasting pan; discard the remaining fat. Set the pan over 2 burners on moderate heat and stir in the flour. Gradually whisk in the stock and the Calvados until smooth. Bring to a boil, whisking, then simmer the gravy over low heat, whisking often, until no floury taste remains, about 8 minutes. Pour in the reserved pan juices and simmer for a few minutes longer. Season with salt and pepper and pour into a gravy boat.