You can always just start with 90 step / mm and then you ask it to move and measure how much it moved.

Say you asked it to move 100 mm and it moved 75mm then you do:

New steps pr mm= 90 * 100/75

As per the equation:

NEW_STEPS_PR_MM = OLD_STEPS_PR_MM * IDEAL_MOVE / REAL_MOVE

Alternatively, you need to measure the diameter of the pulley with the spectra-line and calculate the circumference ( O ) . Then you say: 16 * 360 / (O* 1.8)

The equation being: NEW_STEPS_PR_MM = MICROSTEPS * 360 / ( CIRCUMFERENCE * DEG_PR_STEP)