I was always told that 15 minutes for every pound & a half (1 & 1/2 lbs) at 325 degrees. 30 minutes for every 3 lbs.
So why are they saying 18-20 minutes per lb? Maybe if it's stuff?
First I brined my turkey, than I cook a 16 lb bird for 2 1/2 hours at 325, breast side down than I flip my bird over and cook it for 40 minutes at 450 degrees.
It came out moist/juicey with a nice crisp skin and done all the way to the bone.
Note: It was unstuff. Stuffing takes longer to cook than the turkey dose.