Thank you for you valueable input guys.
I do have Rotorcraft Flight Manual (RFM) of Bell-412. I have also consulted the OEI rate of climb performance graph but could not find the airspeed in straight and level flight.
The helicopter at the time of crash had a gross weight of 10500 pounds, OAT: 34 degree Celsius and altitude : 7000. Though putting these values the on the graph it shows rate of descend of 200 feet per minute. The pilot who survived is giving the statement that he got the sink immediately shutting down one engine. I am unable to comprehend that whether it was the less speed ot it was the rotor RPM which depleted causing the sink and developed high rate of descend ?