Hog, to me the amount of meat the cutters left on spareribs makes a fair bit of difference in calculating cooking time and texture. I always cook ribs at 150F too. I avoid "shiners" and will pick through an entire bunker to find the meatiest racks i can find. I prefer the "St. Louis " racks if i can find them. When done right they can be downright pretty.
All things being equal, which in ribs things seldom are, you might want to shorten up that time a little more, say about 16 hours at 150F to get a tighter bite.
You can judge progress for yourself by pulling each bag of ribs and using your thumb and forefinger pinch alongside the centre of a middle bone to discover the degree of tenderness at that point. Too solid? Then back in they go for a longer cook.