The temperature will be different in different parts of the butt while it cooks. If you're cooking at around 250 degrees, you probably want the temperature in the money muscle to be about 195 when it comes out of the cooker. Taking the temperature in some other part of the butt might result in overcooking the money muscle and make it very difficult to work with. After the butt comes off the cooker, open the foil for 10 to 20 minutes (depending on weather and ambient temperature) so the cooking stops. Then re-wrap loosely and put it in the Cambro (or cooler) until you're ready to slice it. Slice the money muscle off the butt a little behind the fat line using an extremely sharp knife and VERY little pressure on the back stroke only (none on the forward stroke). Use the same knife technique for slicing the muscle. Handle it carefully and everything should turn out fine.
Mister Bob, Pitmaster - Smokestack Lightning, Bad Ass Barbecue. KCBS CBJ #31759 - IMBAS Certified MOINK Baller