In the LabChart Help Menu topic for the ECG Analysis 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).
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