A sett is one complete repeat of the pattern. For example, you could measure the sett in your tartan from one blue overcheck to the next. Alternatively, you could measure from the first white stripe to the next corresponding white stripe where the pattern begins to repeat itself. In that way, measuring sett-width is like measuring wavelength: as long as you measure corresponding points, it doesn't matter where you start.

On your tartan I would suggest centering the blue overcheck in the middle of the outside of the box pleats. The white part of the pattern seems pretty wide and would likely partially "disappear" into the fold as the top of the pleats tapered. You'd have to work up your measurements to see if this would happen, but the blue line would be my definite choice.

Cordially,
David