For my tr8 Z lead screw i want to drive it with belt and use 20t pulley on motor and 40t pulley on the screw. Any advice on how to calculate the step per mm? Thx
with 20t on motor and 40t on the screw it means the motor has to do 2 full circles for the screw to do 1. so the steps per mm would be twice the normal screw steps per mm (as the ratio is 2 to 1).
Use 1: 2 gear ratio.
Ah got it so the pulley ratio basically just a multiplier to the regular step per mm as if there were no pulley?
@Step_Cia try to visualize it this way: imagine the motor rotating 1 circle and then how much the screw will rotate. with a 20/20 T pulley each motor rotation would turn the screw 1 turn - same as the screw being on motor shaft.any ration between the 2 pulleys will affect the screw steps per mm as each motor turn will result in more or less screw turns - but the screw advance is constant regardless of the pulleys sizes.
btw this is also a good way to increase the resolution (albeit by decreasing speed). a 8mm lead screw connected to a motor will result in a 0.02mm adance per full step of the motor (400steps/rotation motor 0.9, ± 5% error, assuming a 0 backlash). add 2 pulleys with a ration 20 motor and 80 on screw and suddenly your full step will advance by 0.005mm. with the lead screw attached to the motor you may get a ± 0.001mm positioning error on the motor shaft while with the belted version will have a ± 0.00025mm error 
so more resolution less speed and I assume I get more torque as well if I do this?
yes to more torque tho for a 3d printer i doubt that should/would be a factor.