I encountered this in the past with a tab supplier whose tab spacing was different than what I was used to. I did the math myself (it was US standard, not metric), something like this:
A4 = 297mm tall
1st tab edge is 1/2" = 12.7 mm from the top edge of sheet; same 12.7 mm non-tab space at the bottom.
297-12.7-12.7 = 271.6 mm
271.6 mm divided by 10 tabs = 27.16 mm width of each of the 10 tabs; each subsequent tab is 27.16 mm down the sheet from the previous one
I then laid this out in InDesign (might have been Quark, showing my age...), sent it to the printer, tweaked as necessary.
For me, this was way easier to manipulate than trying it on CWS.
Good luck!