"Mechanical input from the power lever to an HMU power shaft is summed with a pneumatic pressure signal from the second stage compressor (=P3) to derive a lean (base line) fuel schedule" (Page 7-5 FSM)
Also, the Dash 8-300 series are not equipped with a HMU, but with a MFC.(Page 7-25 FSM)
Do you have the manual from Flightsafety Canada ?