In the first linked post he gives as probably the primary reason the fact that 90º didn't open the pages of the book as much as he desired to capture a good image. He would seem not to be aware of Daniel Reetz's research on the optimum platen angle, which could possibly have been later.
In one of the linked posts, the second I think, he refers to raising the camera, although possibly not for that reason.In any case, as the platen angle is increased, the light position will also need to be moved higher to prevent a reflection of the light source off of the platen surface and into the camera.
It is a shame that David Landin, who put so much effort into developing and documenting his designs, does not seem to have left a description of his final recommendations. He referred in one of his later posts to his preparing a spreadsheet with his revised dimensions, but I don't see it in the above thread, which seems to have ended soon afterwards. I don't know if there are any other relevant threads on which he has posted further details. He hasn't posted for several years now, and although an online search found references to him in Rhyl, England, none were recent.