I just meansued the flange distance, it is indeed 5mm, its 2mm to the top of the dark slide, I have built my 'ground glass' with the use of some 10mm foamcore board (its actually quite solid as im using it for the lens board as well). Basically I will attach some baking paper (the white sort) onto the viewer, then add in a 5mm boarder of foamcore board (still need to purchase) onto it so that it spaces it off the 5mm needed.
I have just basically finished both standards now, I will look at getting one of those macro sliders - probably the 200mm one - and get some captive nuts into the bottom of the standards to screw into.
I am going to be adding some 'bag bellows' which are the alternative to the folding bellows for camera (such as mine) which will have quite a wide lens but also allows any other movements should I want to add any at a later date.
I might also look into adding some sort of handle onto the tripod socket on the base if I do hand hold the camera as an option.