I think I want my writing to reflect how I talk or think about it. So if the spoken sentence is "specification is a half millimetre" then write with ½ instead of "0.5" because the former will be read out correctly.
To you, it’s obvious and natural that „half” is 1/2.
To me, it looks off and I have to „convert” it in my mind to 0.5, which is the way I think about it.