> as 'ex' is bad font that font, you can try 'em' :
>
> \setupbars[overbar][offset=1.2,unit=em,rulethickness=.05]

Thanks Hans, that worked quite well. Not perfect independent from the 
size, as you can see in the attached image, but perfect enough for my 
case, as the sizes differ not soo much (only 1-2pt).



Huseyin