Z motor steps calibration

Hello
I'm a bit confused with the Z Motor steps per mm calibration.
I'm using RRF configurator and when I try to set up the correct steps for the 8mm leadscrew, I only get 1.25mm as pitch possibility.
If I see the specs of this leadscrew, it is 2mm ....
What would be the correct setting ?
Thanks 
@Touchthebitum you will want to take the number of steps per revolution and divide by 2. Since your axis will travel 2mm for every revolution. If you have a 0.9 degree stepper and are not using any microstepping then you would set it to 200 (because 400/2) But I suspect you are using some amount of microsteps the number will be much larger. So take the 400 and multiply it by the microsteps, say for example 64 microsteps then it becomes (400*64)/2 = 12,800.
Hope that helps.

@baird1fa
Thanks
So if I understand correctly, I would have this ? 
@Touchthebitum That looks correct based on the formula for lead screw axis:
Z_steps_per_mm = (motor_steps_per_rev * driver_microstep) / screw_lead

Thanks
Does a leadscrew 8mm (M8) with a 1.25mm pitch exists ?
I don't see anything like this on the net... 
@Touchthebitum standard 8mm threaded rod has a thread pitch of 1.25mm. Was heavily used in the â€˜oldâ€™ RepRap days!
Ian

Thanks for the info