Torque is not a constant for a motor: it varies (a lot) with speed and other factors. Let's consider simplest value: stalling torque -- which is the rotational force it generates while being prevented from turning.
Your calculations omit the efficiency of the motor, which obviously is one of the most important aspects of a motor. You can easily see that the magnetic force generated depends on more than just radius and how much electricity is going in; one of the most important factors is the gap size between the magnetic parts and the exact orientation of the magnetic fields. It also depends on, for example, how much energy is being converted to heat by resistance.
Consider measuring the torque in the most basic way from its definition: torque is force (in newtons) x lever length (in metres).
You can do this by attaching small weights, determining the weight which just stops the motor turning. Or equivalently, by pushing down on a small digital scales (converting to newtons if it is calibrated in grammes or whatever units of mass.)
View along axis of motor
Obviously, there are inaccuracies in any measurement, but this will be sufficient to distinguish, say, a motor with twice as many turns or the effect of varying the voltage on the motor or the strength of the magnets.