Measure with a torque wrench. Normal torque is 18 ft-lb; if you use anti-seize on the threads, reduce this by 30% to about 12 ft-lbs. If you have no torque wrench, then use a socket and socket wrench. After first inserting the plug and tightening by hand as far as you can, use the wrench and just snug it up by holding the wrench down at the neck. Don't gronch on the wrench handle while holding the wrench at the tip.