John, I would've definately checked them sooner than at the five hour mark, cooking over 250. I probably would've checked after about four hours, just to be safe.
You can't just go by the math, using time, weights, and temps with bbq. You've got guestimations, and you check sooner than later. (I'm notorious about overcooking ribs, so I'm also reminding myself!) It's done when it's done, and I hope your bb's didn't overcook. They're much moister and more flavorful before they start to fall off the bone. You want the meat to pull clean from the bone, but not fall off. Hope they turned out good, though!