BBQ'ing on traditional 22" Kettle a 20 lb turkey

Am BBQ'ing on traditional 22" Kettle a 20 lb turkey. Unable to find recipes w/ that large a bird in mind. I have done before but the cooking duration is seemingly erratic. Assuming i add the correct handful of coals per side per hour, can I stick with the 11-14 min per pound rule? Have had these come off in as little as 3.25 hours.

I have done HUNDREDS of birds on a 22 inch kettle...up to 25 lbs, somestimes 2 smaller ones at a time.
I use a stainless bowl in the center filled with steaming/moisturizing juice. beer wine water soy sauce, everything goes in...
I use 2 x chimneys of K-blue around the outside of the pan which is in the middle....you do want to cook at around 300 degrees, keep the water at a low boil, about 6 hrs for a 20 pounder...I use hickory, apple, Jack Daniels oak, cherry, whatever chips I want and use them thruout the 1st 2-3 hours. Poke the coals to get rid of the ash frequently and to keep them hot. Sometimes, usually I have to add another chimney about 2/3 way thru. I used to do a small one every sunday for lunch meet during the week. I cook 2 turkeys for the family T-day yearly, 2 more for the local Vintage Volvo club Christmas party (www.SDVSA.org) and assorted ones thruout the year.

The meat is extremely moist and smoky, no need to brine. anything works for stuffing, coat skin with butter or Evoo.

Thanks much appreciated