st would be the speed of rotation, not the speed of movement.
If it's turning too quickly, divide st by n or - as Azaral suggested - convert to radians.
If it's moving (changing position) too quickly, you'll need to divide sx and sy by n.
n, in this case, is dependent on your computer's speed. It would be trial and error. If you wanted to be able to multiply by a speed variable, you'd need to divide by a higher number.
