Perform calculation with annotation's value

I’m on Rhino 7.

I need to modify the displayed text on a linear dimension to add information to it.
This info would be the result of the subtraction between whatever value the annotation shows and a fixed/known value.

If A is the displayed value and B is the fixed value, then I want to show A (B, D:A-B).
(I think there is a way to show the absolute value, like fabs(), if the result is negative).

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