We have a 45 gallon black tank. After dumping, I take a 3 gallon cat litter bucket, put in 1 cup of Calgon powdered water softener and 1 cup of el cheapo generic powdered laundry soap, fill the bucket with about 2.5 gallons of water, stir to dissolve the powders, then dump it down the toilet (the Geo method). I then refill the bucket with water and dump it down the toilet again. This gives me enough liquid (hopefully) to account for evaporation between trips, and as an added bonus it gets the toilet bowl so clean that I can't look directly at it without sunglasses!
