In the help for the ECG module, the calculation details for the T wave are described as follows:
T wave: A search is made for the first significant peak of either sign, starting from a point after QRS End. [...] If a suitable peak is found, a straight line is fitted by least squares to the tail of the wave, over a data range 70-30% of T Peak. The intersection of this line with the isoelectric level is T end.(From ECG Analysis -> ECG Averaging -> Calculations)
This is the tangent method. The QT interval is calculated as the time between the start of the Q wave (QRS start) and the end of the T wave (T End).