Software problem with stepper motor

[Nikolica]

Before all, I am bad at english.

I have stepper motor and driver for his Big Easy Driver (BED).
Also I have ballscrew with 17 mm movement.
One full step = 1,8* (degrees)
How I can calculate number of impulses for 50, 100, 200 mm..?

I was convert string 'brmmstring' to number 'BRMM', and MAX is number for impulses.

BRMM = atoi(brmmString);

MAX=((int)((BRMM)/0.085));
17/200 = 0,085 is it bad ? My motor is not precesily ..
Thanks in advance

 

[srizbf]

What is the actual movement you got for giving 50mm as distance?

 

[Nikolica]

Now, I can see that my stepper motor make mistakes with steps... I don't know how... I using Big Easy Driver, and sometimes he is precise sometime isn't.

 

[srizbf]

Now, I can see that my stepper motor make mistakes with steps... I don't know how... I using Big Easy Driver, and sometimes he is precise sometime isn't.

that is , sometimes it goes to correct distance ?
sometimes it falls short of it ?
is it ?