Hasselblad used to make a unit for the V system, it is not available any longer and was not made for the H system but the prinicple was so very simple and very accurate. See this link for the instrucrions,
http://www.hasselbladhistorical.org/PDF/HasManuals/Linearmirror.pdf Just putting a mirror on the subject and viewing your own reflection through the camera in the centre on the screen will help.
I think this is what you are asking, am I right?