Hi,
It depends on your hardware, the motor and the mechanical load on the motor.....
therefore i can´t give you a value. Test different values. If you found a solution, then speed it down by at least 25% to get reliable function.
Ramp:
You are trying to speed up the motor form zero to 100% within no time. And you try to slow it down from 100% to zero within no time. This is like driving your car against a solid wall. ;-)
You need a defined acceleration to smoothly speed up from zero speed to 100% speed. And back.
There is a lot of literature around on how to control a stepper motor. Here in the forum, in microcontroller forum or anywhere (google).
Read it carefully.
Klaus