How to calculate torque of a 4-pole BDC hobby motor
You are correct - motor constants asre calculated from shaft speed and you must take into account the number of pole pairs if you use the output frequency of a driven motor to determine speed.  Shaft speed (mechanical rotation) is the basis for calculating torque and velocity constants for motors.  Therefore, for a four-pole motor (or any number of poles) driven by at 1312 rpm, the shaft speed should be used (1312 rpm  = 137 rad/sec).