[Carl n Susan]

Welcome to the MOC!

I moved your question to a new Thread where it can garner the attention it deserves.

As you have quickly learned the gauges are problematical. Some claim to keep them clean with magical elixirs, but most of us take one of two alternatives. A few replace the sensors with external ones that don't get corroded. A fair amount of work as you have to drop the coroplast belly to access the sensors. The rest of us, the vast majority, just ignore the sensors. After a while you will learn how long it takes for the tank to near full given your usage. But that takes some time, so what to do in the mean time? Answer: Listen for the "burp"!.

As the tank nears full, the vent pipe starts to be submerged. Depressing the pedal to flush the toilet results is a little "burp" (best way to describe it) due to a momentary blockage of the vent stack. That is the indicator you are near full.

P.S. *IF* you have a model with a perfectly straight down line from the toilet to the tank, then there is always the flashlight method to check the level. I just don't know of a model that doesn't have 45 degree angles in the pipe.