Accuracy of the information about how much water was added to your system is based on working time of your pump and is assosiated with:
how often system check water level and activate connected pump (one time per 60 is better for accuracy then 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 of water in 30s it means that the reasult is for 30s working pump included start of pump (time from start to work) and amout of water in the pipe. Best for accuracy is to have similar situation - pump will work for 30s.
accuracy of your pump
if the pump has soft start
We have idea how to do better accuracy in this fiels in the future. So please stay tuned.