Your fret measurements should begin at the point where the strings first contact the brass nut. Logically, it would be in the middle of the brass screw, but it's always a good idea to take a peek to be sure.
Once you have determined were the strings first contact the nut, measure out for your first fret. I prefer a MM scale for greater accuracy. To maintain accuracy and avoid "error stacking" measure for each fret from the nut (not from the prior fret). This will really reduce your fret placement error.
Once you have determined where the string contacts the brass bolt/screw, you are in good shape to get frets in the right place.