I used 3 T David's kosher salt for 12 lb turkey. Then friend told me I should have used Diamond Crustal brand as other brands are saltier. So after 5 hours in the David's brine I fully rinsed and started over with about 2 T Diamond brand. Plan on brining for 24 hours.
(Oh, and $10 off your order of $50 or more in the Food52 Shop, too.)