Well according to my notes I installed a Sealand Tankwatch Holding Tank monitor system back in '95 - the type with two extended probes, and one probe just under the lid, all of which are float operated. Our oddly shaped tank was 25 gal, and I calculated that placing one probe down 12" gave 1/3 full, while placing the second down 6" indicated 2/3 full. And it did work, but I wouldn't recommend the system. On the way up, with a clean tank, everything worked just fine. But if you then just emptied the tank without filling it with fresh water up to the probes, the probe floats would often get fouled by the solids, in the down position, and would then sometimes take a long time to work free on the way back "up" so to speak. Which meant that you felt you couldn't really trust the lack of a 1/3 or 2/3 alarm light to mean the tank was mostly empty. More than once I had to undo the wire, unscrew the probe, and soak it in a bucket to get it working again. Trust me, this is NOT something you want to have to do. Filling the whole tank with fresh water after each pump out was the best way to be sure it would work, but who has time at the fuel dock to pump in 25 gallons of water, and then pay for a second pump out?
If I had to do it over again I would for sure look for one of the types that had a sensor strip on the outside of the tank.