Our tank monitor is unreliable, so we put a little counter in the head. Got one of those battery operated ones and stuck it to the wall with Velcro. Every flush is recorded. On our Tecma a full flush is about .6 gallons. Since we have a 40 gallon black water tank, we just round up to a gallon/flush to give us a margin of safety. It is a very convenient back up to the tank monitor and gives you an easy visual of when it is getting close to time to act. For us, when we get into the 30’s it is time to find a place to unload.
The problem is always guests and whether you stuck to the program (“honey? did you remember to hit the counter?”), but once you get into the habit, it works great. As noted in an early post we had a crappageddon or poopocalypse, for want of a better term, and switched to this method. Between the charcoal filter and the counter, all good. No drama. No smells.
Jeff