If you want to be able to set Tabs using a different measurement system, go
to Tools>General and set the Measurement Units to whatever you wish to work
in. The Ruler will be recalibrated accordingly.
Word doesn't make a conversion for you intuitively, however, but if the
Measurement Units is set to Centimeters you can enter the values in Inches
_if_ you type the appropriate designator as well.
For example: If the Measurement units are set to Inches, you can type in the
tab setting as 40pt, 12cm, etc. If the measurement Units is set to Points you
can type the setting in as 3", etc.
HTH |:>)