You will see many posts like,
At 275° a Spatched Turkey will probably take 20 minutes a pound to an IT of 165 in the Breast.
At 225 a Butt will likely take 1.5 to 2 hours a pound to 205.
At 225 a Brisket will usually take 1 to1.5 hours per pound to 195.
These are simple to remember, easy for a Newbie to understand and have many years of Pitmaster's experience that have provided these Estimates.
The biggest misconception....Calling any time per pound a RULE! For instance, 1 to 1.5 hours per pound at 225, to an IT of 195, for Brisket is a GUIDELINE, A TOOL, A TIMING ESTIMATE. It is in No Way an End All, Be All RULE! Now, since this X hours per Pound at Y°F is based on decades and generations of experience using pits of all kinds, these Guidlines are useful to get an " Idea " when to start smoking the meat to be done or close to done, in a specific time frame, like a 5pm Supper.
Of course every piece of meat is different. A Thick and Short 10 pound Porkloin, is going to take a bit longer than a Thinner and Longer 10 pound Porkloin. But, unless you are cooking ahead and don't care how long it takes, Everyone needs some kind timing to go by.
X hours per pound at Y°F until and IT of Z°F, is a Solid Plan for the meats they are intended for. However, it's just an Educated Guess. You still need to monitor the IT, the smoker temp and the Clock. You still need to perform other tests for doneness. Examples, a Guidline for Spare Ribs is 3-2-1 but the Bend Test or the meat is Falling off the Bone, will tell you the Ribs are Done regardless of how long they took. Adjustments with any meat will likely have to be made.
Brisket going long? Wrap and/or crank up the temp.
Brisket going too Fast? Lower the temp, spritz more often to cool the meat or let it finish to 10°F Lower than desired, and cooler it until ready to eat.
There are no Black and White RULES in Smoking Meat. But, to get you started, we have some Great Guidelines you can use to Estimate when to get started the begin Testing for doneness...JJ