Randy, I am not aware of a specific dimension for rail position. In fact, I doubt there is one. The rails are positioned under the top seams. The strap length will determine the one rail location. The actual process typically includes a combined positioning of the top seams and frame during the installation process. Ideally the frame should be adjusted (bent) so the cantrail weatherstrippng conforms to door windows before even starting to install the top material.
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