To calibrate your bottle simply go to Settings and click on "Calibrate bottle" and follow the steps on the screen.
Please know that the bottle uses a time of flight sensor to measure the distance between the cap and the level of water inside the bottle. Despite having selected one of the best sensors on the market, this has a tolerance level of approximately +- 50ml.
We have designed the firmware of the bottle accordingly, so that it does not record small events which could be related to the tolerance of the sensor. This evens out with the use as only the big events are be recorded. For this reason we recommend to use the bottle with a complete refill and then drink to the end so that this works well.
In small words, some little sips might not be recorded immediately but they will update with the next bigger sip.