Your neck scale is what it is. It looks to be nearer a PRS 25" scale than a Gibson or Fender scale.

If anything, with 315mm to the 12th fret, you should be looking at maybe 316-317mm from the centre of the 12th fret to the top E saddle - but you really want to measure it from the neck side of the top E nut slot straight to the top E saddle position. It's hard to tell if you've made a measurement mistake because we can't see both ends of the tape measure in the picture.

I certainly wouldn't trust the metal hook end of the tape measure to be dead accurate, so would use just the tape edge starting at 10mm (or even 100mm) and do a simple subtraction for the overall distance. The 12th fret to saddle length will always be a few mm longer than nut to 12th fret position once the guitar is intonated. Wind the top E saddle all the way forwards and measure between that and the top E nut slot (fretboard face side) as that will be your best case/shortest distance between the nut and saddle that you can achieve.