Butterball advises putting the thermometer in this part of the turkey that provides dark meat
[DOC] [TLE] Check a Turkey's Temperature with a Meat Thermometer | Butterball [PAR] Check Your Turkey's Temperature with Incredibly Easy Thermometer Placement 
... the turkey (near the neck cavity) horizontally to the deepest part of the breast,... [DOC] [TLE] How to Insert a Meat Thermometer - YouTube [PAR] Oct 14, 2014 - 37 sec - Uploaded by ButterballWant to keep your cool while checking your turkey's temperature?