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:

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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.