It is not as bad as I thought. I forgot to take the thickness of the support for the glass in the film holder into account. It added more than a mm to the overall height.
So my last measurment was in fact at about 40 mm above the scanner glass. I've now also tried going down to about 36 mm above the scanner glass, and I can't see any difference from 40 mm. I think the optimal height is between 30 and 40 mm, as noted in the above reference. I may do some more measurements, but if I leave it at about 36 mm, I think it will be more than good enough for scanning 4 x5 negatives.
Bookmarks