Do you mean "servo" motor or "stepper" motor? They are two very different things. I'm not quite sure what kind of stepper motor you have and what kind of modifications you made to it (as most stepper motors are designed to spin 360 degrees.
If it is a "servo" motor you are talking about, sending a 1.5 ms pulse every 20 ms should center the servo causing it to stop. sending a shorter pulse (like 1 ms every 20 ms) will make it rotate in one direction, while sending longer pulses (like 2 ms every 20 ms) will make it rotate the other direction. the closer you get to 1.5 ms pulses should cause the rotation speed to decrease (so sending a 1.4 ms pulse every 20 ms should cause it to rotate slower than sending a 1.2 ms pulse every 20 ms).