I'd run with what you have and take the scale length as 624mm.
If you put that in the StuMac calculator, then you get this:
If you look at your photo with the 12th fret and ruler, then the 10th fret position looks to match up with the calculated 273.8mm (though you could really do with a better ruler with individual, and thinner lined, mm markings, for this type of work.
You can measure a few other frets to confirm, but if they match, then I'd run with 624mm as the scale length and position the neck accordingly.