Your pic shows a nut to centre-of-saddle distance of roughly 648.5mm. As the design scale length is 25.5” or 647.7mm, you are a lot closer than you think to being spot-on. It’s the nut to 12th fret distance that appears to be 1mm too short for the design scale length and is throwing your measurements out.

Can you take a photo of that measurement to double check?