From what I've learned and put into practice in my backyard kind of way - I'm with Smoke on this...As those ribs reach the right temp and texture, the meat pulls away from the bones and leaves the ends exposed. I believe if you have about 9/16ths showing, you should be in good shape...could even go to 11/16ths.
If you aren't going to use foil, use a mop sauce or a spray to help maintain moisture. I watch a lot of the Grill Masters and the ones that aren't using foil are mopping every 30 minutes like Capn Jack sez.
OH...a major change I made in my routine was made after watching Aaron Franklin smoke ribs. He emphasized the need to allow the ribs to REST a while before slicing for the same reason you allow steak to rest. He recommends waiting until you can hold the ribs in your bare hands - keeping the ribs tented, not wrapped. It made a difference for us. Just sayin'...
B T W, I found an idea that works great on unfoiled ribs. Mix apple juice (or apple cider vinegar) with grenadine in a spray bottle and spray the ribs - meat side up - about every 20 minutes after the meat begins to pull away. The Juice adds a little flavor and the grenadine adds sweetness AND lays down a beautiful reddish glaze.
Keep in mind now...I'm just a backyard jack. My disclaimer can be found on page 6 of my contract with LTBBQ.