i've never measured temp on ribs - when the meat starts to pull in a bit from the end of the bones and they pass the bend test, they are good to go. at 250 for 4 hours is usually good for me. the bend test = pick up the slab with a pair of tongs and bounce them slightly. If they are ready, the slab will bow until the meat starts to crack on the surface. A small crack means you need a little more time. It should be close to breaking when you lift the slab. You'll get the feel for this with practice. i got this from another site as it explains it easier than i could. hope this helps.
Al - CG SuperPro w/SFB and Outlaw w/SFB, 18" WSM, 22.5" WSM