You will need to know the magnetizing current of the motor. It is the actual current drawn when the motor is at no-load. Or, the manufacturer can provide it. And you know the full load amps from the motor nameplate.
Set up a vector triangle with the magnetizing current on one side and the full load current as the hypotenuse. Solve for the remaining side which is the full load torque producing current.
This is the torque producing current at full load torque. It is directly proportional to actual torque. So, given the actual torque, find the actual torque producing amps. Using the Pythagoreon Theorem again, use the known magnetizing amps as one side, the actual torque producing amps as the other side, and solve for total amps (the hypotenuse). That is the total amps under reduced torque conditions.