I'm preparing an 8# butt roast for pulled pork.

For dry brining, the salt penetrates meat about 1 cm (~1/2 inch) per 24 hours. For a larger piece of meat, you'll want to dry brine several days.

I agree with AntoniaJames that this recipe will be easier to handle by cutting the larger piece in half. The basic premise with cooking meat is that it reaches target internal temperature. A larger piece of meat will take more time than a smaller piece.

By cutting the 8# piece in half, you should have three roughly equivalent sized pieces and the heat will penetrate them at a fairly equal rate.

However, ultimately you should use a meat thermometer and pull each piece out when it reaches your target temperature. If you don't you'll likely have undercooked and overcooked pieces.

One interesting technique is to pull the pieces out as they reach the desired temperature, wrap them in foil, and hold them in an insulated cooler (like a styrofoam or plastic ice chest). The meat will stay in a temperature zone that will help to continue breaking down the collagen and soften the meat without overcooking (and thus drying out).

But for sure, use a meat thermometer. It will give you the most reliable and replicable results.

It will work best if you cut the total quantity of pork into at least three pieces. I usually cut the meat into 4 - 5 inch chunks.

Roasting at 300 degrees, it takes about 3 hours, but the best way to know when it's ready is by the internal temperature. You want the internal temperature to be a bit higher than for most pork dishes, because the higher temperature will break down the collagen in the pork. That will improve the pork's flavor and texture immeasurably. I aim for at least 190 degrees Fahrenheit.

Also, I cover the pork for most of the process, to keep it from drying out too much. During the last half hour or 45 minutes, I uncover it, to let some of the edges crisp up a bit. ;o)