Your sheer horizontals will be fabricated to the exact lengh ordered, when the vanes are in the open position. When the vanes are rotated to the closed position, the shading will become 3/8" longer.
When closed on a inside mount, a slight puckering may occur on the lower most sheer section because of the contact with the sill. You may slightly raise the shading off the window sill. If preferred, the shading can be ordered 3/8" less than the measured length.