If you are taking a look at a Hale panel, then you would measure from down inside the bottom track then all the method as much as the leading track, stopping short of going inside the leading track. Determine how wide the flap requires to be. Open your moving glass door wide enough for your family pet to come through easily.
There's your minimum width. Do not purchase This Author that isn't at least that broad. Identify the flap height and step-over measurements you need. Preferably, the top of the flap will be at least as high as the top of the family pet at his 'withers' (top of his shoulders). That method, he doesn't have to crouch to use the pet door.
That's just how much he has to step over. So, flap height plus step-over is the height of the top of the flap. Typically, the lower the step-over, the much better. Specifically when your family pet is older or arthritic, so you may choose a flap 17" high with a 3" step-over to one with a 15" height and 5" step-over.
More flap and less step-over can likewise be practical if you've got a small family pet that needs to show a bigger one. A family pet door that includes a user-adjustable step-over makes it simpler and offers more versatility for the future. These are only available with Endura Flap animal panels.
Many brands are offered in white, satin (aluminum color) or bronze (really dark brown). You can paint any of these to get a various color if you wish, but the easiest to paint is white. Consult your local paint buy guide and paint recommendations. The product in which you are installing the insert does not affect the real setup or efficiency of the animal panel so much as the measurements.
Make sure to examine these things prior to purchase. See if you have dual-pane glass. You probably do and, unless you remain in a Very temperate climate area, you'll wish to pick a pet door that has dual-pane glass likewise. You can tell by positioning a finger on both sides of your glass.