I like to fill the ring. For ribs, it will be more than you need, but you'll know you won't run out and have to refill. When the cook is done, shut all vents and there should be plenty left to use up on the next cook. A side benefit of doing this is you'll learn if you cooker is tight because if it is not, the fire will not fully extinguish and most of the charcoal will burn up.