Pi * (diameter of the shaft + diameter of the string) = TotalDiameter

Steps per revolution * microstep / TotalDiameter = Step per mm

Having a 5 mm shaft and 0.33 mm string and 200 steps per revolution with 1/16 microstepping the math is:

3.1415*(5+0.33)=16.744195
200*16/16.744195=191.11100892 or 191.11 (the firmware only takes 2 digits)

And for instance

200*16/17.08976=187.246631901 or 187.24

5mm shaft 0.5 string:

3.1415*(5+0.5)=17.27825
200*16/17.27825=185.203941371 or 185.20

3.1415*(10+0.3)=32.35745
200*16/32.35745=98.895308499 or 98.89

And..

here is some knot science: