How fortunate that you can see down to the tank in the toilet! I have an elbow that prevents that.
In your situation, I would buy one of those ultrasonic distance measurement tools.
https://www.amazon.ca/Ultrasonic-Distance-Measurer-Estimator-Measuring/dp/B004IYNAHE/ref=sr_1_2?ie=UTF8&qid=1474673056&sr=8-2&keywords=Distance+meter
With one or two physical measurements with a stick you can find out what distances indicate empty and full enough to leak out the tank cleaning port.
None of my tank level lights work. I get the freshwater tank level accurately with a clear plastic tube attached to the drain tap. The tube is long enough to extend up to the top of the rear bumper, which happens to be the tank full level.
