Not sure about 20 mm pitch too :) Belt and pulley can offer more possibilities, good idea! Do you know where I could find good ones?

Someone made a graph for me for the same motor with different pitch considering the weight of the gantry, ballscrew type, motors specs,... :
Click image for larger version.  Name: SpeedCuttingForceGraph.png  Views: 408  Size: 9.8 KB  ID: 23121

10mm seems to be a good compromise. What do you think about that?