How to calculate torque of the motor?
With a 200 g motor, 400 mW power consumption sounds awfully low. It sounds like you are running the motor at no load.
If you want to measure or estimate the torque into a load by the mechanical power = electrical power method, then you need to put a load on the motor. Suitable loads include a fan, another motor/generator driving resistors for a load, or a friction or electromagnetic brake.
Assuming 100% efficiency would give you an upper bound on mechanical power output. Given the motor size, it's likely to be much less than that.
Once loaded with a suitable mechanical load, you'll need to measure or estimate the motor speed, as the output power in watts is torque (Nm) times angular speed (radians/s). Conversion between rpm or rev/s to rads/s is fairly straightforward.
Once you have measured voltage V, current I and motor speed ω when loaded, and estimated your expected efficiency η, your torque at that load and current will be
torque = ηVI/ω
If you can actually measure the torque, by mounting the load on a balance, or by winding a weight up round a drum, then a more interesting calculation is to calculate the efficiency.