You are right, there are errors.
The figure usually quoted for a motor is the ON LOAD speed at a given voltage. What would be far more useful is the no load speed figure but there is no reason why the manufacturers name plate would give you such a thing.
The no load speed will be higher as there is little discrepancy between emf and applied voltage. The implication of this is that you need a higher speed than the nameplate rating to generate an emf equal to the rated terminal voltage.
With many motors with good speed regulation the difference may only be about 10% but if you happened to choose a motor with large speed droop on load you could be off by a lot more.
Typically I would increase the makers plate speed rating by about 15% and work from that as a first guide. If it looks hopeless reject the motor. If it seems reasonable then you would be justified in actually taking a voltage reading when driven at a known speed ( even hand cranking and timing). That will get you an exact figure to calculate cut in speed. You still need to know about its internal resistance to know how it will load with speed.
Flux
