I haven't done spares in a long time. We prefer loin backs (baby backs). The best I've found are from Sam's Club. Costco sells Swift which has a large portion of the "white meat" loin still attached which always turns out dry and tough. Second best is BJ's. Anyway, at 225, mine usually take 6 - 6 1/2 hours (no preheat). I always start them bone side down and flip and rotate them about halfway through, just seems to get a better bark on top. I start testing (toothpick test) at 5 1/2 hours and as soon as they pass I pull them and if dinner is going to be later, I double wrap them in foil and put them in a 200F oven, but for no longer than about 1/2 hour so they don't dry out. They are usually bite-off-the-bone tender and juicy.
Your timing at 250 matches mine, I just like the result at 225 better. Let me know what you think if you try it.