How can I estimate cooking time for a brined and roasted turkey? For previous Thanksgivings, we have brined and roasted our turkey.  (Here is the detailed process.)  After brining for about a day, we roast the bird for 30 minutes at 500&deg;F and then bake it at 350&deg;F until it's reached 160&deg;F internally.  

The bird is stuffed with an apple and half an onion (sliced), a cinnamon stick and some sage leaves for flavor.  During the high-heat phase, the breast is protected with a foil sheet.  The turkey legs are pulled inward towards the body; we do not spatchcock it.

The problem is that it's always taken much longer than we'd estimated for the turkey to come to temperature.  We end up serving dinner 2 or 3 hours later than expected which is certainly sub-optimal.  Based on the turkey's weight (and preparation process), how can we estimate how long it will take to cook?  

We do use an independent thermometer to verify oven temperature.

Ideally, I'd like a formula for how to calculate cooking time from weight, and how to adjust based on details such as brining.
I've never roasted an entire turkey, but my grandmother followed Delia Smith's advice every year and it never failed us. Typically it worked at at around 15 - 20 minutes per lb of bird.


  
  Cooking times for other sizes of turkey: 
  
  
  8-10 lb (3.5-4.5 kg) – 30 minutes at the high temperature, then 2½-3
  hours at the lower temperature, then a final 30 minutes (uncovered) at
  gas mark 6, 400°F (200°C).  15-20 lb (6.75-9 kg) – 45 minutes at the
  high temperature, then 4-5 hours at the lower temperature, then a
  final 30 minutes (uncovered) at gas mark 6, 400°F (200°C).