I too cut my racks in half. You will no longer experience the bottom one or two ribs being burnt (the burnt bottom ribs happened to me too the first time I made them in the PBC). Additionally, I put both half racks on one hook. You could still get 8 racks of ribs in the PBC this way.
Also, you may consider adding a touch of brown sugar or turbinado sugar to your mix. The sugar will help create a caramelizing for the ribs that will give it a nice char. For spare ribs I have cooked right at the 3 hour mark on multiple occasions. At this point I'm able to get serious separation off the bone with the meet if I pull on the bone hard enough. That's the rule of thumb for ribs being done.
I always do exactly 20 minutes. even if some of the charcoal has not burned to the level of others. Keep in mind with the lid on and bars in place you are creating an almost humid oven like atmosphere in the PBC. The temperature is evenly distributed in there even if the charcoal has not burned evenly. 20 minutes with the lid off gets the fire temp to a certain point and then cools down to the 275-300 range (generally speaking and will vary by humidity, wind, elevation) once the lid is on. Leaving the lid off longer will get the fire significantly hotter and take much longer to get to the ideal cook range.