Question about Cat Mate Microchip cat flap 360W

Unfortunately, nothing is documented about the cut-out and no template is available. (I just had a look at the operating instructions).
Since I had installed the flap in the cellar door and didn't want to worry about breaking too much, I ordered this flap, unpacked it, held it up to the door and drew in the outline to cut it out, cut it out with the saw and then enlarged the cut-out again because the flap didn't fit straightaway.

Now, in retrospect, I would first make a template from the cardboard or thin wooden panel (craft plywood), which I would then use to draw the cut-out in the door.