My ECw seems to be quite off for my substrate sensor

ECw is a calculation heavily depending on VWC and ECb, if any of them is not fit with the calibration profile ECw will be off. If the ECw is off it is best to look at the trend of ECb on the same level of VWC. For instance: If water is given every day, reading the ECb on a high value VWC in the day can be the benchmark of ECb for the next day.