This page provides some slightly more technical information, as well as another perspective on where to measure the height of your door.  

Lower Guide Rail (double or single Dreamscreen)

This image shows the number of different ways the Dreamscreen lower guide rail can be mounted, based on the existing doorframe and threshold.  This applies to face and flush mounts  (53 Kb)

Cross section of a face mount

The face mount is the preferred installation on single Dreamscreens, but may be used with double (French) doors as well.  In this case, the Dreamscreen mounts to the surface (or face) of the existing door molding.

(35 KB)

Face mount housing/jamb placement

This page shows the different possible placement for the housing and jamb on the existing door molding.  Because the Dreamscreen design is so flexible, a number of equally viable options exist. (54 KB)

FLUSH MOUNT DIAGRAMS

Cross section of a flush mount

The flush mount is the preferred installation on double Dreamscreens, usually with French doors, but may be used with single doors as well.  In this case, the Dreamscreen mounts to the inside of the existing door frame. (57 KB)

Because the flush mount is inside the existing door frame, the location of the lower guide rail is pretty much set by the door itself.  Take a look at Lower Guide Rail (double or single Dreamscreen).