Where did you get your figures? (I don't doubt them) My calculations, from the designer at pinhole.cz came to .282 mm, but I figured .3 was about as precise as I could get without specialized tools.
One problem I have found with my design, is that not all 4x5 film holders are created equal. I built my camera around a holder with a pronounced ridge that keeps the holder in place when you pull the darkslide. Without the tension of a spring back, some holders are prone to slipping out of place. I already have an idea or two to fix things; this has been a tremendous learning experience.