The standoffs are just long enough to make the sail tight, i.e. no remarkable bending. The purpose of vents is to pass some of the wind pressure through the sail and reduce the pull and speed of the kite. Sun screen mesh is pretty tight and does not stretch at 'diagonal' direction. However it is porous enough to reduce the pull of the kite in high winds. Templates are for the final size of the sail panels.
All seams joining sail panels are straight.
The length of the lower spreader is 0.76 m. The lenght of the upper spreader determines the shape of the upper part of the sail.