Add some water to the black tank before you get to the CG. Trying to extend the days a black tank will last by being stingy with water can be a bad idea and lead to maybe only a couple of days of usage. Unless you have a really good idea how your tank performs, add 3-5 gallons of water before using the tank and be generous with the amount of water you flush with. Some tanks are quite shallow and if you try and conserve water, it doesn't take long for a pyramid of p**p to build up and block the toilet.