The manufacture of my trailer had the wrong size listed (it is smaller than they said). If you really want to know for sure, measure the tank size and calculate or pour water in it. Advantage of adding water is you can get a relative calibration of your monitor (add water and watch the monitor transition). Of course, this calibration will be subjective to how clean your sensors are, but it's better than nothing. Did the same for my fresh water tank too. Now I have a fairly accurate monitor for the fresh tank and a much better "guess" for the grey and black.
BTW, you can make a black water tank dipstick (old broom stick with calibration marks on it) that will always be accurate (as long as there is no pile)
