pulley ratios on 3 pulley axis

[John S]

I see, that makes the last picture make more sense. Since it is therefore the belt that is moving, not the pulley/stepper you need to use the circumference of the 24T pulley to work out the steps per unit distance. Since the belt is 5mm pitch, and 24T pulley, one revolution moves the axis 120mm ... so including the reduction that's effectively 26.66..mm pitch.

Nothing to do with the circumference.
24 teeth, 5mm pitch 1 rev = 24 x5 = 120mm per rev
can't be anything else.