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I normally use the 2-2-1 method when I do baby backs just because they are smaller and contain less meat than the spares. Its basically just a guideline, you will eventually be able to tell just by looking at them when its time to move to the next step.
Do a thread search and you will find lots of info regarding your question.
Agree with Joe, 2-2-1 for bb and they are just an estimate. With practice you can just take a peek and know by the pullback of the meat and the droop of the rack when you pick it up.
Actually I like 2 ½ - 1 1/2 - ½ for BB. Alot depends on how much fall off the bone you like. Less time in stage two will have them have alittle more bite to the rib.
I didn't make tender ribs until I tossed the timer and watched for the "Pull". I smoke at 225/250 until 1/4" pull, wrap and splash until 1/2-3/4" pull. unwrap and sauce for "awhile" (45-60 minutes)
Like more meat on the ribs..so I use spares more than BB's. 3-2-1 is the way to go...apple wood and apple juice in the pan. I do 210 as a guideline depending on the outside temp. Here in Ohio that can very greatly depending on Mother Nature.