Text formatting: Tab stops and center?

I'd like to center a text using the middle tab stop. My scale is in mm and I cannot find the "right" position in order to place my text precisely in the middle.

On the picture, the top one is formatted with the tab stop and the bottom one with the "center" align text (cannot be used for setting center and right text on the same line). You can see the step with the red arrow.

Is there any way to define the exact center of the width ?

Same problem when I want to place a text in the middle inside a cell of a table which contains several columns on the same line.

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