I give another try at this forum...
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?
I know that Hasselblad device...
the method I suggested above has two advantages...
you can have it now and for little money and 2nd.
you can place the mirror with the hole ( this is done by scratching the mirror
coating away so that you can look thru ) in front of the lens, or the front standard,
rear standard, if using a MF SRL remove the back and place the mirror instead.
If you want to give the mirrors a nice touch cover them with matt black vinyl and cut
a circle out, you will have then nice concentric circles when parallel alinement
is achieved.
btw a digital spirit level is also pretty useful.
Hey, pretty nice !
Why don't you show as a picture of your setup ? Would be more illustrative...
;)