Whether oven, smoker, or deep fryer this type of gizmo is your best friend:
https://www.thermoworks.com/DOT
There are cheaper ones but the probes tend to be very fragile so careful.
Stick that probe in the boob, set the alarm for 155 F for a 18lb bird (degree or two less for a big bird), and drink beer until your gizmo says you have to work again. Leave the probe in, set the bird on the counter and now set the alarm to 160. It'll finish on the counter. Then wait 15 or so to carve. Whatever wacky mad science beer-can-up-the-ass Turducken recipe you're doing, follow those instructions and you will not have dry Turkey.