Hey Willem, you need to measure to about the centre of the 12th fret, so I measured nut to 12th fret about 312/313mm. Glue will fill a gap of 1-1.5mm at the heel end of the neck.
Can you please double check if that 319-320mm distance is from the centre of the 12th fret ? If you measured from the closest edge of the 12th fret then this distance will increase slightly to the centre of the 12th fret.
Do you have a 500 or 600mm steel ruler ? generally they are more accurate than a tape measure and as Brendan pointed out if using a tape measure I'd start at say 100mm as the tip can move or give inaccurate readings