Depends on the installation, the Herald and HS748 were power boosted,(in general, depends on model) ie water meth injection gave a torque indication above isa dry power, the F27 (later series) was power restored, this to bring the power up to isa dry power when oat is above isa, water meth was not used to restore power lost due to altitude.
The injection system was pretty basic, the control was not exact, in practice the extra water meth induced power tended to surge above the dry torque setting on restored engines with a rise of RPM above 15000 and when run during ground runs both RPM and torque tended to return to settings quite quickly, during take off the prop would be coarsening and the watermeth control unit would be controlling with a slight lag, this causing a fluctuating torque.
BR om15