As said 321 411, 221, it isn't about temp it is about time, at about 2-3 hours start checking for how much pull back you have, and take a set of tong and lift a whole rack there should be some nice flex not stiff. On pull back I start thinking about foil or the next stage when there is 1/4" pull back on most of the ribs. When you foil the ribs, they will still be cooking and may reach 1/2" pull back while in the foil. When you open the foil packet, try and pull off the last bone on the smaller end, you will know how done they are from this test.