I would think that the moment of inertia of the spool would create some discrepancy in the measurement, so the weight does matter. I'm also not sure how you figure that "half" the weight of an end-attached rod should be added to the bucket weight, but you might know something I don't. Back in college the physics classes all seemed harder than that
Suffice to say I think you should give the bucket a push to get it moving and see if the motor can sustain the motion to determine if it's capable of the tested torque -- static friction could cause havoc in this measurement.