Torque = Power / rotorspeed.
Power in W, rotorspeed in radians/sec, torque in Nm.
If you're going to use hp and rpm in the equation, it'd become
Torque = ( Power / 1.341 ) / ( rotorspeed x 0.1047 ) x 1000
Still gives you torque in Nm though. If your cockpit gauges are in Nm you're in business but I bet they're in %. Will check a cab next time I'm over to move the Chippy....
If gauges are in % you'd need to know the 'datum' torque ie how many Nm of torque = 100%. That'll be a question for Westlands - unless on-site engineering has that figure.