You said picnic roast, so first off, if the skin is on, I would peel it off, trim off the fat under the skin to no more than 1/8" or so. My favorite rub for pork is to mix Simply Marvelous Sweet and Spicy with about 25% medium gring black pepper and coat the entire thing. Let it sit for an hour. Meanwhile, I fire my UDS or Kettle to 225F, with the vents set to reach 275F or so. I put the meat on at 225F, close it up and let the cooker run, after a couple of hours, it will be 275F or a little higher, let it go.
If you have a put thermo, use it as a guideline, when it gets to 185F to 19oF internal temperature, I will probe it, usually it is not quite done, the probe meets resistance. Once the probe goes in easily, I will grab a piece and pull, if it comes from the shank easily, then I figure it is done. Wrap in foil, cooler rest for 2 hours. Pull.
I like to take the foil drippings, defat them using a separator, then mix with equal part of vinegar. Then dump that into and equal part of BBQ sauce. Shake it up, serve in a squirt bottle. My pork tastes good just like that.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."