For spares, I don't go by time at all...the 3-2-1 is just a guideline...I gauge the smoke stage by pull-back and/or the slab bend test. At the 3-hr mark, you should give them a peek, and if you see good pull-back, then do the bend test.
When I see at least 1/4" of pull-back (bone-ends sticking out), then it's probably time for the foiling. When in doubt, lift the slab from the middle...it should resist natural bending, and only sag a little on the ends. This is caused by the muscle fiber shrinkage and tightening during the cooking...more bend (flexibility) = less cooked.
I have one other trick up my sleeve for foiling...I use a baking pan and put a foil tent over it...this way I can pull the pan out of the smoker and peek inside for additional pull-back after steaming a couple hours. The pan also gives me a leak-free steaming...no worries about bones poking through the foil.
If you're looking for fall off the bone, wait until you see at least 1/2" or more total of pull-back after the steam, then if you want to firm up the bark again, carefully lift the slab out of the pan, supporting it's length well, and place back onto the open smoker grate for 30-60 minutes, depending on how much bark you like. I always lay the ribs with the bone ends down, meat-side up.
Once you fully understand what each process of the 3-2-1 does for your ribs, you can play with the variables and get exactly what you want every time...or you can change it now and then if you get bored with the texture. The visual checks of the rib meat shrinkage/pull-back are your best gauge to judge how much they have been cooked, as time will vary greatly with different smokers...hot/cold
spots on the grate, chamber temp fluctuations, high elevation, smoke chamber humidity, among many other factors, all can effect the overall cooking time.
Since I've done so many spares in the past 12-18 months using the ~3-2-1 method (I never foiled 'til I came to SMF), I can get bone-popping ribs, or tender, needing a twist to break the bone free, or I can leave 'em so they have teeth marks when you take a bite.
My version of the 3-2-1 for 5.25-5.5lb (pre-trimmed) slabs of spares is generally closer to 4-2.5-0.5 @ 225*, for a twist-able bone, and a bit less smoke with more foil time for bone-popping ribs.
Hope that gives you a better feel for what to look for.