Accuracy of the information about on how much water was added to your system is based on the working time of your pump and is assosiated associated with:
how often system check the water level and activate the connected pump (one time per 60 is better for accuracy then than one time per 10 min)
how long is the pipe from pump to your sump (shorter pipe better accuracy, we need to understand that pipe in this case after added water is always empty) - during calibration you have amout the amount of water in the 30s it means that the reasult result is for 30s working pump included the start of the pump (time from start to work) and amout amount of water in the pipe. Best The best for accuracy is to have a similar situation - the pump will work for the 30s.
accuracy of your pump
if the pump has a soft start
We have an idea of how to do better accuracy in this fiels these fields in the future. So please stay tuned.
...