Fixed height/position character underline that does not vary with font size

[Chas]

Is is possible to create an underline to a glyph character that is a fixed height.  i.e. does not scale with the font size?  This underline also needs to be consistently (x)mm below the font baseline irrespective of the font size.

If anyone could help me with resolving this problem I would be very grateful.

Many thanks