On a Full Packer .. I always have my probe in the Flat like Ron. I "START" checking the point when the Flat probe reads 190' for probe friction. The other thing is what temp are you cooking at? My normal is 225-235' with no farkin peakin to add to the long cook time as it is...
P.S. - If you just want sliced ... my humble opinion is to just cook flats if you are serveing sliced.
Big Jim - Newnan, GA
Customized Lang Model 84 Longneck