I still think of this as Jon Singer’s Turkey Algorithm, because that’s the title under which I knew it for decades; but Singer says it was devised by the late Mike Fellinger, and must be credited to him. It goes:
“For a turkey of greater than ten pounds, the roasting time should be equal to 1.65 times the natural log of the weight of the bird in pounds, cooked at 325 F.”
If you’re not a person who normally calculates natural logs, go to Google. Say you have a 20-pound turkey. Type in natural log 20 and hit the search button. Google will tell you that the natural log of 20 is 2.99573227. Multiply this by 1.65. The result will be 4.9429582455, or five hours.