Like others, I do not use a bag. I use foil and follow the guidelines on this site:
Let's Talk Turkey
Now that you have ascertained that your oven cooks lower than you thought, I think you're in a good position to use this guide and expect fairly accurate results!
ALSO, my hubby and I always freak out over Thanksgiving so we have started cooking the turkey the day before and putting the servings in a slow cooker with juice on Thanksgiving day so the meat is moist and the house smells like turkey for the guests. This way, I know the turkey has been cooked, it isn't dry and the juice from the slow cooker is used to make gravy. Additionally, my oven remains free for biscuits and all the other things that need baked on the day.
Good luck - I hate Thanksgiving stress so I am there with you! x