MDSsecondary support length

Dear ALL,

While placing a support for cable tray, the GENSEC length is automatically set. i need to know how can i modify it to get the right length. Bellow an example :

When the distance between the centerline of the cable tray and the center of the vertical GENSEC is equal to 275mm, the whole length for the horizontal bar should be 600m but I'm finding it 590 (10mm are missing). 

Do you have any idea how to solve this.

Regards.